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/auto-commit-on-exit.ts

@@ -1,49 +0,0 @@
-/**
- * Auto-Commit on Exit Hook
- *
- * Automatically commits changes when the agent exits.
- * Uses the last assistant message to generate a commit message.
- */
-
-import type { HookAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: HookAPI) {
-	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");
-		}
-	});
-}