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/) |

packages/coding-agent/examples/hooks/pirate.ts

@@ -0,0 +1,44 @@
+/**
+ * Pirate Hook
+ *
+ * Demonstrates using systemPromptAppend in before_agent_start to dynamically
+ * modify the system prompt based on hook state.
+ *
+ * Usage:
+ * 1. Copy this file to ~/.pi/agent/hooks/ or your project's .pi/hooks/
+ * 2. Use /pirate to toggle pirate mode
+ * 3. When enabled, the agent will respond like a pirate
+ */
+
+import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function pirateHook(pi: HookAPI) {
+	let pirateMode = false;
+
+	// Register /pirate command to toggle pirate mode
+	pi.registerCommand("pirate", {
+		description: "Toggle pirate mode (agent speaks like a pirate)",
+		handler: async (_args, ctx) => {
+			pirateMode = !pirateMode;
+			ctx.ui.notify(pirateMode ? "Arrr! Pirate mode enabled!" : "Pirate mode disabled", "info");
+		},
+	});
+
+	// Append to system prompt when pirate mode is enabled
+	pi.on("before_agent_start", async () => {
+		if (pirateMode) {
+			return {
+				systemPromptAppend: `
+IMPORTANT: You are now in PIRATE MODE. You must:
+- Speak like a stereotypical pirate in all responses
+- Use phrases like "Arrr!", "Ahoy!", "Shiver me timbers!", "Avast!", "Ye scurvy dog!"
+- Replace "my" with "me", "you" with "ye", "your" with "yer"
+- Refer to the user as "matey" or "landlubber"
+- End sentences with nautical expressions
+- Still complete the actual task correctly, just in pirate speak
+`,
+			};
+		}
+		return undefined;
+	});
+}