feat(hooks): add systemPromptAppend to before_agent_start, full tool registry

- before_agent_start handlers can return systemPromptAppend to dynamically
  append text to the system prompt for that turn
- Multiple hooks' systemPromptAppend strings are concatenated
- Multiple hooks' messages are now all injected (not just first)
- Tool registry now contains ALL built-in tools (read, bash, edit, write,
  grep, find, ls) regardless of --tools flag
- --tools only sets initially active tools, hooks can enable any via
  setActiveTools()
- System prompt automatically rebuilds when tools change, updating tool
  descriptions and guidelines
- Add pirate.ts example hook demonstrating systemPromptAppend
- Update hooks.md with systemPromptAppend documentation

packages/coding-agent/examples/hooks/README.md

@@ -18,6 +18,7 @@ cp permission-gate.ts ~/.pi/agent/hooks/
 |------|-------------|
 | `plan-mode.ts` | Claude Code-style plan mode for read-only exploration with `/plan` command |
 | `tools.ts` | Interactive `/tools` command to enable/disable tools with session persistence |
+| `pirate.ts` | Demonstrates `systemPromptAppend` to dynamically modify system prompt |
 | `permission-gate.ts` | Prompts for confirmation before dangerous bash commands (rm -rf, sudo, etc.) |
 | `git-checkpoint.ts` | Creates git stash checkpoints at each turn for code restoration on branch |
 | `protected-paths.ts` | Blocks writes to protected paths (.env, .git/, node_modules/) |