feat: optional PATH argument for host, serve, mcp, and watch commands

[sattva1]

Summary

Closes #65.

Adds an optional [PATH] positional argument to watch, host, serve, and mcp commands, matching the existing analyze pattern. This allows running Axon against a project directory without cd-ing into it — critical when Axon is installed globally and the target project has its own isolated virtualenv.

Changes

• src/axon/mcp/server.py — New set_db_path() setter following the existing set_storage()/set_lock() injection pattern. Allows the CLI to configure the MCP server's database path before entering the event loop.

• src/axon/cli/main.py:

  • watch, host, serve, mcp accept path: Path = typer.Argument(Path("."), ...) with early directory validation
  • _run_shared_host() takes repo_path as a parameter instead of hardcoding Path.cwd()
  • _start_host_background() passes the path as an explicit positional argument to the subprocess
  • watch() refactored to delegate to _initialize_writable_storage() (was duplicating ~15 lines of init logic)
  • serve() calls set_db_path() before the watch/no-watch branch, so both code paths honor the path
  • ui() updated to pass repo_path to _run_shared_host() (API contract change, no new argument)

• tests/cli/test_main.py — 10 new tests: help text visibility, non-existent path rejection, set_db_path() global update, subprocess command verification.

Usage

# Index from anywhere
axon analyze /home/user/projects/my-app

# Run server commands against the same project — no cd required
axon host --watch /home/user/projects/my-app
axon serve --watch /home/user/projects/my-app
axon mcp /home/user/projects/my-app
axon watch /home/user/projects/my-app

Backward compatible — all commands default to . (current directory).