feat(coding-agent): add hook API for CLI flags, shortcuts, and tool control

Hook API additions:
- pi.getTools() / pi.setTools(toolNames) - dynamically enable/disable tools
- pi.registerFlag(name, options) / pi.getFlag(name) - register custom CLI flags
- pi.registerShortcut(shortcut, options) - register keyboard shortcuts

Plan mode hook (examples/hooks/plan-mode.ts):
- /plan command or Shift+P shortcut to toggle
- --plan CLI flag to start in plan mode
- Read-only tools: read, bash, grep, find, ls
- Bash restricted to non-destructive commands (blocks rm, mv, git commit, etc.)
- Interactive prompt after each response: execute, stay, or refine
- Shows plan indicator in footer when active
- State persists across sessions

packages/coding-agent/CHANGELOG.md

@@ -36,7 +36,9 @@
 
 - `$ARGUMENTS` syntax for custom slash commands as alternative to `$@` for all arguments joined. Aligns with patterns used by Claude, Codex, and OpenCode. Both syntaxes remain fully supported. ([#418](https://github.com/badlogic/pi-mono/pull/418) by [@skuridin](https://github.com/skuridin))
 - Hook API: `pi.getTools()` and `pi.setTools(toolNames)` for dynamically enabling/disabling tools from hooks
-- New example hook: `plan-mode.ts` - Claude Code-style read-only exploration mode with `/plan` command
+- Hook API: `pi.registerFlag(name, options)` and `pi.getFlag(name)` for hooks to register custom CLI flags
+- Hook API: `pi.registerShortcut(shortcut, options)` for hooks to register custom keyboard shortcuts
+- New example hook: `plan-mode.ts` - Claude Code-style read-only exploration mode with `/plan` command, `--plan` flag, and Shift+P shortcut
 
 ### Changed