Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

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

@@ -0,0 +1,44 @@
+/**
+ * Pirate Extension
+ *
+ * Demonstrates using systemPromptAppend in before_agent_start to dynamically
+ * modify the system prompt based on extension state.
+ *
+ * Usage:
+ * 1. Copy this file to ~/.pi/agent/extensions/ or your project's .pi/extensions/
+ * 2. Use /pirate to toggle pirate mode
+ * 3. When enabled, the agent will respond like a pirate
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function pirateExtension(pi: ExtensionAPI) {
+	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;
+	});
+}