refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

browser_cli/remote/registry.py

@@ -0,0 +1,53 @@
+"""Persistence for remembered remote browser endpoints and key specs."""
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+
+from browser_cli.constants import CONFIG_DIR
+from browser_cli.endpoints import _normalize_endpoint
+
+REMOTE_REGISTRY_PATH = CONFIG_DIR / "remotes.json"
+
+def load_remotes() -> dict[str, dict[str, str]]:
+  if not REMOTE_REGISTRY_PATH.exists():
+    return {}
+  try:
+    data = json.loads(REMOTE_REGISTRY_PATH.read_text(encoding="utf-8"))
+  except Exception:
+    return {}
+  if not isinstance(data, dict):
+    return {}
+  # Normalize keys so old entries stored as "domain:443" match current lookups.
+  return {_normalize_endpoint(str(endpoint)): cfg for endpoint, cfg in data.items() if isinstance(cfg, dict)}
+
+def is_valid_key_spec(value: str) -> bool:
+  """Return True for 'agent', 'agent:<selector>', or a plausible key file path."""
+  return value == "agent" or value.startswith("agent:") or (
+    not value.startswith("<") and ("/" in value or Path(value).suffix in {".pem", ".key"})
+  )
+
+def save_remote_key(endpoint: str, key_spec: str) -> None:
+  """Persist the key spec (e.g. 'agent' or a file path) for a remote endpoint."""
+  if not endpoint or not key_spec or not is_valid_key_spec(key_spec):
+    return
+  remotes = load_remotes()
+  current = remotes.get(endpoint, {})
+  current["key"] = key_spec
+  remotes[endpoint] = current
+  REMOTE_REGISTRY_PATH.parent.mkdir(parents=True, exist_ok=True)
+  fd = os.open(str(REMOTE_REGISTRY_PATH), os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+  with os.fdopen(fd, "w", encoding="utf-8") as f:
+    f.write(json.dumps(remotes, indent=2, sort_keys=True))
+
+def key_for_remote(endpoint: str | None) -> str | None:
+  if not endpoint:
+    return None
+  cfg = load_remotes().get(endpoint) or {}
+  key = cfg.get("key")
+  if not key:
+    return None
+  key_str = str(key)
+  # Reject corrupted values (e.g. str(AgentKey(...)) saved by an older bug).
+  return key_str if is_valid_key_spec(key_str) else None