fix(gemini): use AfterTool instead of PostToolUse for Gemini CLI hooks

[Tibsfox]

Summary

Gemini CLI uses AfterTool as the post-tool hook event name, not PostToolUse (which is Claude Code's event name). The installer was registering the context monitor under PostToolUse for all runtimes, causing Gemini to print an "Invalid hook event name" warning on every run and silently disabling the context monitor hook.

The comment at install.js:2034 even noted the assumption: "Gemini shares same hook system as Claude Code for now" — this turned out to be incorrect for hook event names.

Changes

• bin/install.js: Introduce postToolEvent variable that resolves to AfterTool when runtime === 'gemini', PostToolUse otherwise. Used for both the install and registration paths.
• bin/install.js (uninstall): Clean up both PostToolUse and AfterTool entries for backward compatibility — users who installed with the old PostToolUse key need their stale entries removed on uninstall/reinstall.
• hooks/gsd-context-monitor.js: Runtime-aware hookEventName in output metadata (detects Gemini via GEMINI_API_KEY env var).
• docs/context-monitor.md: Document both event names, add Gemini-specific manual registration example.

Verification

1. Install GSD for Gemini: npx get-shit-done-cc --gemini --global
2. Check ~/.gemini/settings.json — context monitor should be under hooks.AfterTool, not hooks.PostToolUse
3. Run gemini -p "ping" --output-format text — no "Invalid hook event name" warning
4. Uninstall — both AfterTool and PostToolUse entries cleaned up

Backward Compatibility

• Existing Gemini installs with PostToolUse entries: The uninstall path now cleans up both event names, so reinstalling will migrate to AfterTool automatically.
• Claude Code / OpenCode installs: No change — they continue using PostToolUse.

Closes #750

🤖 Generated with Claude Code