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/hooks/git-checkpoint.ts

@@ -1,53 +0,0 @@
-/**
- * Git Checkpoint Hook
- *
- * Creates git stash checkpoints at each turn so /branch can restore code state.
- * When branching, offers to restore code to that point in history.
- */
-
-import type { HookAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: HookAPI) {
-	const checkpoints = new Map<string, string>();
-	let currentEntryId: string | undefined;
-
-	// Track the current entry ID when user messages are saved
-	pi.on("tool_result", async (_event, ctx) => {
-		const leaf = ctx.sessionManager.getLeafEntry();
-		if (leaf) currentEntryId = leaf.id;
-	});
-
-	pi.on("turn_start", async () => {
-		// Create a git stash entry before LLM makes changes
-		const { stdout } = await pi.exec("git", ["stash", "create"]);
-		const ref = stdout.trim();
-		if (ref && currentEntryId) {
-			checkpoints.set(currentEntryId, ref);
-		}
-	});
-
-	pi.on("session_before_branch", async (event, ctx) => {
-		const ref = checkpoints.get(event.entryId);
-		if (!ref) return;
-
-		if (!ctx.hasUI) {
-			// In non-interactive mode, don't restore automatically
-			return;
-		}
-
-		const choice = await ctx.ui.select("Restore code state?", [
-			"Yes, restore code to that point",
-			"No, keep current code",
-		]);
-
-		if (choice?.startsWith("Yes")) {
-			await pi.exec("git", ["stash", "apply", ref]);
-			ctx.ui.notify("Code restored to checkpoint", "info");
-		}
-	});
-
-	pi.on("agent_end", async () => {
-		// Clear checkpoints after agent completes
-		checkpoints.clear();
-	});
-}