fix: XML-escape dynamic values in launchd plist (#181)

[leanworld7-netizen]

Summary

The macOS start_with_launchd() function writes dynamic values ($agent_path, $auth_url, $auth_client_id, $auth_audience, $autospin_url, $MONK_AGENT_LOCAL, $MONK_PLUGIN_VERSION, $host, $agent_path_env) directly into the launchd plist XML using a heredoc without XML escaping.

If any of these values contain XML-special characters (&, <, >, ", '), the resulting plist is malformed XML that launchd silently rejects — the agent never starts and no error is surfaced to the user.

Fix

Adds an xml_escape() shell function that performs the five required XML entity replacements (& → &, < → <, > → >, " → ", ' → ') and applies it to all dynamic values before they are interpolated into the plist heredoc.

The launchd_configured() grep checks are also updated to match against the escaped values, ensuring the idempotency check correctly detects an already-configured plist.

Testing

• Verified that paths containing & (e.g., /tmp/agent&fixture/monk-agent) are properly escaped to /tmp/agent&fixture/monk-agent in the plist XML
• The escaped plist is valid XML that launchd accepts
• Paths without special characters are unaffected (escaping is a no-op)

Fixes #181

Bounty eligibility: I have signed up at monk.io and used the product.

[leanworld7-netizen]

Linking to bounty issue #181. This fix addresses the reported vulnerability. Ready for review.