refactor(api): namespaced SDK + dedicated transport layer

Restructure the Python API and internals around composable namespaces and
a standalone transport/endpoint layer. Bump to 0.12.0.

Python API:
- Replace flat methods (b.tabs_list(), b.group_list()) with namespaces:
  b.nav, b.tabs, b.groups, b.windows, b.dom, b.extract, b.page, b.storage,
  b.cookies, b.session, b.perf, b.extension.
- Shrink browser_cli/__init__.py to a thin composition root; move all
  behaviour into browser_cli/sdk/ (one module per namespace + factories,
  base, routing).

Internals:
- Add browser_cli/transport.py and remote_transport.py to isolate IPC from
  command logic; client.py now delegates instead of owning transport.
- Add browser_cli/endpoints.py for endpoint resolution and
  browser_cli/errors.py for shared error types.
- Extract markdown rendering into browser_cli/markdown.py (out of extract).
- Add USER_AGENT to version_manager.

Tooling & tests:
- Add justfile with common dev tasks.
- Update CLI commands and demo to the namespaced API.
- Rework tests for the new layout; add test_transport.py and
  test_refactor_boundaries.py to lock in module boundaries.

BREAKING CHANGE: flat API methods are removed in favour of namespaces
(e.g. b.tabs_list() -> b.tabs.list(), b.group_list() -> b.groups.list()).

README.md

@@ -290,88 +290,100 @@ from browser_cli import BrowserCLI
 b = BrowserCLI()
 ```
 
-Every CLI command has a corresponding SDK method. The call blocks until the browser responds and returns the data directly as a Python object.
+Commands are grouped into namespaces on the client (`b.tabs`, `b.dom`, `b.session`, ...). Each call blocks until the browser responds and returns the data directly as a Python object.
 
 ```python
-# Navigation
-b.open("https://example.com")
-tab = b.open_tab("https://example.com")      # returns a bound Tab object
-tab = b.open_tab("https://example.com", wait=True, timeout=10)
-b.open("https://example.com", background=True)
-b.open("https://example.com", window="work")
-b.reload()
-b.hard_reload()
-b.back()
-b.forward(tab_id=1234)
-b.focus_url("github")
+# Navigation  ── b.nav
+b.nav.open("https://example.com")
+b.nav.open("https://example.com", background=True)
+b.nav.open("https://example.com", window="work")
+b.nav.reload()
+b.nav.hard_reload()
+b.nav.back()
+b.nav.forward(tab_id=1234)
+b.nav.focus("github")
+b.nav.to(1234, "https://example.com")   # navigate a specific tab in place
+b.nav.search("google", "python asyncio")
 
-# Tabs
-tabs = b.tabs_list()          # list[Tab]; in multi-browser mode each tab.browser is set
-tabs = b.tabs()               # short alias for tabs_list()
-active = b.active_tab()       # active Tab object
-tab = b.tab(1234)             # tab by ID
-tab = b.find_tab("github")    # first matching tab or None
-tabs = b.find_tabs("github")  # alias for tabs_query()
-b.tabs_active(1234)
-b.tabs_close(1234)
-b.close_tab(tab)              # accepts Tab or tab ID
-b.tabs_close_inactive()
-b.tabs_close_duplicates()
-b.tabs_filter("youtube")      # list of matching tabs
-b.tabs_query("pull request")
-counts = b.tabs_count("github")  # int, or BrowserCounts(total=..., by_browser=...) in multi-browser mode
-html = b.tabs_html()          # full HTML string of active tab
-b.tabs_sort(by="domain")
-b.tabs_merge_windows()
-b.tabs_dedupe()
+# Tabs  ── b.tabs
+tabs = b.tabs.list()          # list[Tab]; in multi-browser mode each tab.browser is set
+tab = b.tabs.open("https://example.com")              # returns a bound Tab object
+tab = b.tabs.open("https://example.com", wait=True, timeout=10)
+active = b.tabs.active()      # active Tab object
+tab = b.tabs.get(1234)        # tab by ID
+tab = b.tabs.first("github")  # first matching tab or None
+b.tabs.activate(1234)
+b.tabs.close(1234)
+b.tabs.close(tab_ids=[1, 2, 3])  # close many in one round-trip (IDs or Tab objects)
+b.tabs.close_inactive()
+b.tabs.close_duplicates()
+b.tabs.filter("youtube")      # list of matching tabs
+b.tabs.query("pull request")
+counts = b.tabs.count("github")  # int, or BrowserCounts(total=..., by_browser=...) in multi-browser mode
+html = b.tabs.html()          # full HTML string of active tab
+b.tabs.sort(by="domain")
+b.tabs.merge_windows()
+b.tabs.dedupe()
 
 # Bound Tab helpers
-tab = b.active_tab()
+tab = b.tabs.active()
 tab.pin()
 tab.screenshot()
 tab.refresh()
 tab.wait_for_load(timeout=10)
 tab.watch_url(r"/done$")
 
-# Tab groups
-groups = b.group_list()       # list[Group]; in multi-browser mode each group.browser is set
-groups = b.groups()           # short alias for group_list()
-b.groups_create("research")  # plural alias for group_create()
-b.group_create("research")    # creates group, returns Group
-b.group_close(42)
-b.group_tabs(42)              # tabs inside a group
-b.group_count()               # int, or BrowserCounts(...) in multi-browser mode
+# Tab groups  ── b.groups
+groups = b.groups.list()      # list[Group]; in multi-browser mode each group.browser is set
+b.groups.create("research")   # creates group, returns Group
+b.groups.close(42)
+b.groups.tabs(42)             # tabs inside a group
+b.groups.add_tab(42, "https://example.com")
+b.groups.count()              # int, or BrowserCounts(...) in multi-browser mode
 
-# Windows
-windows = b.windows_list()    # in multi-browser mode each dict has a "browser" key
-b.windows_rename(1, "work")
-b.windows_open()
-b.windows_open("https://example.com")
-b.windows_close(1)
+# Windows  ── b.windows
+windows = b.windows.list()    # in multi-browser mode each dict has a "browser" key
+b.windows.rename(1, "work")
+b.windows.open()
+b.windows.open("https://example.com")
+b.windows.close(1)
 
-# DOM  (active tab must be http/https)
-elements = b.dom_query("h2")           # list of { tag, text, attrs }
-texts = b.dom_text(".article p")       # list of strings
-attrs = b.dom_attr("a", "href")        # list of strings
-exists = b.dom_exists(".cookie-banner")# bool
-b.dom_click(".accept-button")
-b.dom_type("#search", "hello world")
-b.wait_for_selector("#results", visible=True, timeout=10)
+# DOM  ── b.dom  (active tab must be http/https)
+elements = b.dom.query("h2")           # list of { tag, text, attrs }
+texts = b.dom.text(".article p")       # list of strings
+attrs = b.dom.attr("a", "href")        # list of strings
+exists = b.dom.exists(".cookie-banner")# bool
+b.dom.click(".accept-button")
+b.dom.type("#search", "hello world")
+b.dom.wait_for("#results", visible=True, timeout=10)
+b.dom.eval("document.title")
 
-# Extract
-links = b.extract_links()     # list of { text, href }
-images = b.extract_images()   # list of { alt, src }
-text = b.extract_text()       # string
-data = b.extract_json("#app-data")  # parsed Python object
+# Extract  ── b.extract
+links = b.extract.links()     # list of { text, href }
+images = b.extract.images()   # list of { alt, src }
+text = b.extract.text()       # string
+data = b.extract.json("#app-data")  # parsed Python object
+md = b.extract.markdown("article")
 
-# Sessions
-b.session_save("before-meeting")
-b.session_load("before-meeting")
-sessions = b.session_list()   # [{ name, tabs, savedAt }, ...]
-b.session_remove("before-meeting")
-diff = b.session_diff("session-a", "session-b")
+# Page / storage / cookies
+info = b.page.info()
+b.storage.set("token", "abc")
+val = b.storage.get("token")
+cookies = b.cookies.list(domain="example.com")
+
+# Sessions  ── b.session
+b.session.save("before-meeting")
+b.session.load("before-meeting")
+sessions = b.session.list()   # [{ name, tabs, savedAt }, ...]
+b.session.remove("before-meeting")
+diff = b.session.diff("session-a", "session-b")
 # diff = { "added": [...urls], "removed": [...urls] }
-b.session_auto_save(True)
+b.session.auto_save(True)
+
+# Performance + extension
+b.perf.status()
+b.perf.set_profile("gentle")
+b.extension.reload()
 
 # Misc
 clients = b.clients()
@@ -385,7 +397,7 @@ from browser_cli import BrowserCLI, BrowserNotConnected
 
 b = BrowserCLI()
 try:
-    tabs = b.tabs_list()
+    tabs = b.tabs.list()
 except BrowserNotConnected:
     print("Browser is not running or extension is not loaded")
 except RuntimeError as e:
@@ -397,11 +409,11 @@ from browser_cli import BrowserCLI, BrowserCounts
 
 b = BrowserCLI()
 
-tabs = b.tabs_list()
+tabs = b.tabs.list()
 for tab in tabs:
     print(tab.browser, tab.title)
 
-counts = b.tabs_count()
+counts = b.tabs.count()
 if isinstance(counts, BrowserCounts):
     print(counts.total)
     print(counts.by_browser)