feat: use Hammerspoon URL scheme for click-to-focus

[iFwu]

Summary

• Fix shell escaping bug: The current create_focus_command builds a complex multi-line Lua script and passes it through shlex.quote() → terminal-notifier -execute. The nested quoting (single quotes in Lua, shell escaping, terminal-notifier's own shell execution) causes the script to fail, triggering the "Could not restore window focus" error notification on every click.
• Improve click-to-focus performance (~50%): Each notification click currently spawns a new hs CLI process with ~1.8s startup overhead, making total click-to-focus take ~3.4s. The URL scheme is handled in-process by Hammerspoon, reducing latency to ~1.6s (just the macOS Space switch animation).
• Simplify code: Replaces 30+ lines of inline Lua script generation with a single URL string construction.

Changes

cc_notifier.py

• Replace create_focus_command(window_id) -> list[str] with create_focus_url(window_id) -> str that returns a hammerspoon://focus?id=<window_id> URL
• Change send_notification to use -open <url> instead of -execute <shell_command>
• Remove import shlex (no longer needed)

README.md

• Update Hammerspoon init.lua instructions: replace hs.window.filter + hs.timer requires with hs.urlevent and the focus URL event handler

Backward Compatibility

Fully backward compatible. Without Hammerspoon installed, focus_window_id is never set (the get_focused_window_id() call raises and sets window ID to "UNAVAILABLE"), so the -open code path is never reached.

Test plan

• Verify notification appears correctly when user switches away from Claude Code window
• Verify clicking notification focuses the correct window across macOS Spaces
• Verify click-to-focus latency improvement (expected ~3.4s → ~1.6s)
• Verify no "Could not restore window focus" error notifications appear
• Verify behavior without Hammerspoon installed (graceful degradation, no -open flag sent)

[trentmcnitt]

I've been playing with this for the past 30 minutes and I can't get my window to focus when using the PR. Does cross-space window focusing work for you (including when you have multiple app windows open -- i.e. multiple VS Code projects in separate windows and spaces)?

What OS + Hammerspoon versions are you on?