Add optional path argument to host, serve, and mcp commands

[sattva1]

Problem

axon analyze accepts an optional [PATH] argument, allowing it to index a project from anywhere:

axon analyze /home/user/projects/my-app

This creates .axon/ storage inside the target project. However, host, serve, and mcp commands hardcode Path.cwd().resolve() as the repo path (cli/main.py:490, cli/main.py:927), so they can only operate on the project you're currently inside.

This creates a problem when Axon is installed globally (or in a separate environment) and the target project uses its own isolated virtualenv (e.g., via pyenv). You can analyze the project from outside, but you can't run host --watch or serve against it — and you can't run them from inside the project dir because axon isn't on that venv's PATH.

Proposed Solution

Add an optional [PATH] argument to host, serve, and mcp, matching the existing analyze signature:

axon host --watch /home/user/projects/my-app
axon serve --watch /home/user/projects/my-app
axon mcp /home/user/projects/my-app

When provided, use that path instead of Path.cwd() for storage resolution, file watching, and all repo-scoped operations.

Scope

Small change — thread the optional path argument through:

• host → _run_shared_host() → repo_path parameter (currently hardcoded at line 490)
• serve → repo_path (currently hardcoded at line 927)
• mcp → mcp_main() → _load_storage() (already accepts repo_path, just needs the CLI plumbing)

_load_storage() already handles an optional repo_path parameter with a Path.cwd() fallback (line 54), so the storage layer is ready — it's just the CLI entry points that need the argument.