chore: remove dead code identified by vulture + coverage cross-validation

Remove unused code confirmed dead via vulture scan, grep verification,
and coverage analysis:

- _get_bridge_dir (cli/commands.py): 82-line function with zero callers
- add_assistant_message (agent/context.py): method body never executed,
  also removed now-unused build_assistant_message import
- _tool_parameters_schema (agent/tools/base.py): redundant copy of schema
  already exposed via the `parameters` property
- MSTEAMS_REF_TTL_S (channels/msteams.py): unused constant (production
  uses config.ref_ttl_days directly); inlined in test
- MESSAGE_TYPE_USER (channels/weixin.py): unused constant

nanobot/cli/commands.py

@@ -1280,90 +1280,6 @@ def channels_status(
     console.print(table)
 
 
-def _get_bridge_dir() -> Path:
-    """Get the bridge directory, setting it up if needed."""
-    import hashlib
-    import shutil
-    import subprocess
-
-    # User's bridge location
-    from nanobot.config.paths import get_bridge_install_dir
-
-    user_bridge = get_bridge_install_dir()
-    stamp_file = user_bridge / ".nanobot-bridge-source-hash"
-
-    # Find source bridge: first check package data, then source dir
-    pkg_bridge = Path(__file__).parent.parent / "bridge"  # nanobot/bridge (installed)
-    src_bridge = Path(__file__).parent.parent.parent / "bridge"  # repo root/bridge (dev)
-
-    source = None
-    if (pkg_bridge / "package.json").exists():
-        source = pkg_bridge
-    elif (src_bridge / "package.json").exists():
-        source = src_bridge
-
-    if not source:
-        console.print("[red]Bridge source not found.[/red]")
-        console.print("Try reinstalling: pip install --force-reinstall nanobot")
-        raise typer.Exit(1)
-
-    def source_hash(root: Path) -> str:
-        digest = hashlib.sha256()
-        for path in sorted(root.rglob("*")):
-            if not path.is_file():
-                continue
-            rel = path.relative_to(root)
-            if rel.parts and rel.parts[0] in {"node_modules", "dist"}:
-                continue
-            digest.update(rel.as_posix().encode("utf-8"))
-            digest.update(b"\0")
-            digest.update(path.read_bytes())
-            digest.update(b"\0")
-        return digest.hexdigest()
-
-    expected_hash = source_hash(source)
-    current_hash = stamp_file.read_text().strip() if stamp_file.exists() else None
-
-    # Reuse only a bridge built from the currently installed source.
-    if (user_bridge / "dist" / "index.js").exists() and current_hash == expected_hash:
-        return user_bridge
-
-    if (user_bridge / "dist" / "index.js").exists() and current_hash != expected_hash:
-        console.print(f"{__logo__} WhatsApp bridge source changed; rebuilding bridge...")
-
-    # Check for npm
-    npm_path = shutil.which("npm")
-    if not npm_path:
-        console.print("[red]npm not found. Please install Node.js >= 18.[/red]")
-        raise typer.Exit(1)
-
-    console.print(f"{__logo__} Setting up bridge...")
-
-    # Copy to user directory
-    user_bridge.parent.mkdir(parents=True, exist_ok=True)
-    if user_bridge.exists():
-        shutil.rmtree(user_bridge)
-    shutil.copytree(source, user_bridge, ignore=shutil.ignore_patterns("node_modules", "dist"))
-
-    # Install and build
-    try:
-        console.print("  Installing dependencies...")
-        subprocess.run([npm_path, "install"], cwd=user_bridge, check=True, capture_output=True)
-
-        console.print("  Building...")
-        subprocess.run([npm_path, "run", "build"], cwd=user_bridge, check=True, capture_output=True)
-        stamp_file.write_text(expected_hash + "\n")
-
-        console.print("[green]✓[/green] Bridge ready\n")
-    except subprocess.CalledProcessError as e:
-        console.print(f"[red]Build failed: {e}[/red]")
-        if e.stderr:
-            console.print(f"[dim]{e.stderr.decode()[:500]}[/dim]")
-        raise typer.Exit(1)
-
-    return user_bridge
-
-
 @channels_app.command("login")
 def channels_login(
     channel_name: str = typer.Argument(..., help="Channel name (e.g. weixin, whatsapp)"),