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/dirty-repo-guard.ts

@@ -0,0 +1,56 @@
+/**
+ * Dirty Repo Guard Extension
+ *
+ * Prevents session changes when there are uncommitted git changes.
+ * Useful to ensure work is committed before switching context.
+ */
+
+import type { ExtensionAPI, ExtensionContext } from "@mariozechner/pi-coding-agent";
+
+async function checkDirtyRepo(
+	pi: ExtensionAPI,
+	ctx: ExtensionContext,
+	action: string,
+): Promise<{ cancel: boolean } | undefined> {
+	// Check for uncommitted changes
+	const { stdout, code } = await pi.exec("git", ["status", "--porcelain"]);
+
+	if (code !== 0) {
+		// Not a git repo, allow the action
+		return;
+	}
+
+	const hasChanges = stdout.trim().length > 0;
+	if (!hasChanges) {
+		return;
+	}
+
+	if (!ctx.hasUI) {
+		// In non-interactive mode, block by default
+		return { cancel: true };
+	}
+
+	// Count changed files
+	const changedFiles = stdout.trim().split("\n").filter(Boolean).length;
+
+	const choice = await ctx.ui.select(`You have ${changedFiles} uncommitted file(s). ${action} anyway?`, [
+		"Yes, proceed anyway",
+		"No, let me commit first",
+	]);
+
+	if (choice !== "Yes, proceed anyway") {
+		ctx.ui.notify("Commit your changes first", "warning");
+		return { cancel: true };
+	}
+}
+
+export default function (pi: ExtensionAPI) {
+	pi.on("session_before_switch", async (event, ctx) => {
+		const action = event.reason === "new" ? "new session" : "switch session";
+		return checkDirtyRepo(pi, ctx, action);
+	});
+
+	pi.on("session_before_branch", async (_event, ctx) => {
+		return checkDirtyRepo(pi, ctx, "branch");
+	});
+}