feat: add Oh My Pi (omp) support

[obviousbread]

Closes #52.

What this does

Adds Oh My Pi (omp) — the @oh-my-pi/pi-coding-agent CLI — as
a fourth supported tool alongside Claude Code, Codex, and Gemini CLI. cn on omp,
cn off omp, and cn status now behave like the existing tools.

Why omp needs a different mechanism

The Claude/Codex/Gemini integrations register a hook command by editing a
config file (settings.json / config.toml). omp doesn't read hook commands
from a config file — it loads JS/TS extension modules from
~/.omp/agent/extensions/. So enabling = writing a small managed extension
module, disabling = removing it.

cn on omp writes ~/.omp/agent/extensions/code-notify.js, which subscribes to
omp's agent_end event and forwards it to the existing notifier.sh
(stop omp <project>). Because the contract into notifier.sh is unchanged, all
existing delivery — sound, voice, Slack/Discord, click-through, the global mute
kill switch, and rate limiting — is reused as-is.

Notable design decisions

• Outcome-aware: the extension maps the final assistant stopReason so
  errored/aborted runs notify as errors instead of a false "Task Complete". It
  scans back to the last assistant message, since the array can end on a
  tool-result/bash/compaction message that would mask the outcome.
• Global mute keeps working: the project name is passed via cwd (not as the
  notifier's positional project arg), so basename($PWD) still scopes the text
  while cn off / the kill switch still suppress the notification.
• Binary-only detection (command -v omp), matching detect_codex /
  detect_gemini_cli. A directory probe on ~/.omp/agent false-positives:
  code-notify creates the extensions/ subdir on enable, and it survives an omp
  uninstall.
• Safe enable/disable: enable refuses to overwrite a non-managed file at that
  path; disable removes only the managed file and never touches ~/.omp/agent
  (omp's data dir).

Scope

macOS / Linux, completion event only (mirrors the Codex scope). Windows omp and a
needs-input/idle event are left as follow-ups — status/voice wiring already warns
that omp is unsupported on Windows.

Files

• lib/code-notify/utils/detect.sh — detect_omp + registry
  (get_installed_tools, is_tool_installed)
• lib/code-notify/core/config.sh — omp paths, generate_omp_extension,
  is/enable/disable_omp_hooks, dispatch in enable_tool / disable_tool /
  is_tool_enabled
• lib/code-notify/core/notifier.sh — omp display name
• lib/code-notify/commands/global.sh — enable/disable/status wiring +
  supported-tools text
• lib/code-notify/utils/help.sh, lib/code-notify/utils/voice.sh — omp listed;
  per-tool voice status/teardown
• scripts/install-windows.ps1 — warns omp unsupported on Windows
• README.md — Features, usage table, How It Works
• tests/test-omp-extension.sh (+ scripts/run_tests.sh wiring) — functional JS
  harness for the extension's runtime contract, plus mute / refuse-overwrite /
  binary-only detection coverage

Testing

make test: the new omp extension support test passes, along with 25 of the
other 26 tests. The single failure — click-through resolver
(click_through_resolve_default_term_program's builtin reverse-lookup fallback)
— is pre-existing and unrelated to this change: it fails identically on
main (2b854f6), and this branch touches no click-through code. Happy to open a
separate issue for it.

[mylee04]

@obviousbread thanks for your effort. I like your suggestion and just merged it.