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/auto-commit-on-exit.ts

@@ -0,0 +1,49 @@
+/**
+ * Auto-Commit on Exit Extension
+ *
+ * Automatically commits changes when the agent exits.
+ * Uses the last assistant message to generate a commit message.
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("session_shutdown", async (_event, ctx) => {
+		// Check for uncommitted changes
+		const { stdout: status, code } = await pi.exec("git", ["status", "--porcelain"]);
+
+		if (code !== 0 || status.trim().length === 0) {
+			// Not a git repo or no changes
+			return;
+		}
+
+		// Find the last assistant message for commit context
+		const entries = ctx.sessionManager.getEntries();
+		let lastAssistantText = "";
+		for (let i = entries.length - 1; i >= 0; i--) {
+			const entry = entries[i];
+			if (entry.type === "message" && entry.message.role === "assistant") {
+				const content = entry.message.content;
+				if (Array.isArray(content)) {
+					lastAssistantText = content
+						.filter((c): c is { type: "text"; text: string } => c.type === "text")
+						.map((c) => c.text)
+						.join("\n");
+				}
+				break;
+			}
+		}
+
+		// Generate a simple commit message
+		const firstLine = lastAssistantText.split("\n")[0] || "Work in progress";
+		const commitMessage = `[pi] ${firstLine.slice(0, 50)}${firstLine.length > 50 ? "..." : ""}`;
+
+		// Stage and commit
+		await pi.exec("git", ["add", "-A"]);
+		const { code: commitCode } = await pi.exec("git", ["commit", "-m", commitMessage]);
+
+		if (commitCode === 0 && ctx.hasUI) {
+			ctx.ui.notify(`Auto-committed: ${commitMessage}`, "info");
+		}
+	});
+}