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/docs/hooks.md

@@ -775,6 +775,44 @@ pi.setTools(["read", "bash", "edit", "write"]);
 
 Only built-in tools can be enabled/disabled. Custom tools are always active. Unknown tool names are ignored.
 
+### pi.registerFlag(name, options)
+
+Register a CLI flag for this hook. Flag values are accessible via `pi.getFlag()`.
+
+```typescript
+pi.registerFlag("plan", {
+  description: "Start in plan mode (read-only)",
+  type: "boolean",  // or "string"
+  default: false,
+});
+```
+
+### pi.getFlag(name)
+
+Get the value of a CLI flag registered by this hook.
+
+```typescript
+if (pi.getFlag("plan") === true) {
+  // plan mode enabled via --plan flag
+}
+```
+
+### pi.registerShortcut(shortcut, options)
+
+Register a keyboard shortcut for this hook. The handler is called when the shortcut is pressed.
+
+```typescript
+pi.registerShortcut("shift+p", {
+  description: "Toggle plan mode",
+  handler: async (ctx) => {
+    // toggle mode
+    ctx.ui.notify("Plan mode toggled");
+  },
+});
+```
+
+Shortcut format: `modifier+key` where modifier can be `shift`, `ctrl`, `alt`, or combinations like `ctrl+shift`.
+
 ## Examples
 
 ### Permission Gate