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()).

justfile

@@ -0,0 +1,81 @@
+# browser-cli — task runner
+# Run `just` or `just --list` to see all recipes.
+
+# Default browser profile for `install` / quick checks
+browser := "brave"
+
+# Show available recipes
+default:
+    @just --list
+
+# ── Setup ──────────────────────────────────────────────────────────────
+
+# Install Python deps + Node deps for the extension build
+setup:
+    uv sync
+    npm ci
+
+# Register the native messaging host for a browser (default: {{browser}})
+install browser=browser:
+    uv run browser-cli install {{browser}}
+
+# ── Python ─────────────────────────────────────────────────────────────
+
+# Run the full Python test suite
+test:
+    uv run pytest -q
+
+# Run tests verbosely
+testv:
+    uv run pytest -v
+
+# Run a single test file or node id, e.g. `just test-one tests/test_api.py`
+test-one target:
+    uv run pytest -v {{target}}
+
+# Run tests with a coverage report
+cov:
+    uv run pytest --cov=browser_cli --cov-report=term-missing
+
+# ── Extension (TypeScript / MV3) ───────────────────────────────────────
+
+# Build the extension bundle (extension/background.js)
+build:
+    npm run build:extension
+
+# Run the JS (node:test) extension tests
+test-ext:
+    npm run test:extension
+
+# Full extension gate: tsc + build + node --check + JS tests
+check:
+    npm run check:extension
+
+# ── Everything ─────────────────────────────────────────────────────────
+
+# Run the complete gate: extension check + Python tests
+ci: check test
+
+# ── Release ────────────────────────────────────────────────────────────
+
+# Show the current version
+version:
+    @grep -m1 '^version' pyproject.toml | cut -d'"' -f2
+
+# Bump version in pyproject.toml AND extension/manifest.json, e.g. `just bump 0.10.5`
+bump new:
+    sed -i 's/^version = ".*"/version = "{{new}}"/' pyproject.toml
+    sed -i 's/"version": ".*"/"version": "{{new}}"/' extension/manifest.json
+    @echo "bumped to {{new}} — remember to update uv.lock (uv lock) and rebuild the extension"
+
+# Verify both version files agree
+version-check:
+    @py=$(grep -m1 '^version' pyproject.toml | cut -d'"' -f2); \
+     ext=$(grep -m1 '"version"' extension/manifest.json | cut -d'"' -f4); \
+     if [ "$py" = "$ext" ]; then echo "ok: $py"; else echo "MISMATCH pyproject=$py manifest=$ext"; exit 1; fi
+
+# ── Demos ──────────────────────────────────────────────────────────────
+
+# Run the Python SDK demo
+demo:
+    uv run python examples/demo.py