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/protected-paths.ts

@@ -1,30 +0,0 @@
-/**
- * Protected Paths Hook
- *
- * Blocks write and edit operations to protected paths.
- * Useful for preventing accidental modifications to sensitive files.
- */
-
-import type { HookAPI } from "@mariozechner/pi-coding-agent";
-
-export default function (pi: HookAPI) {
-	const protectedPaths = [".env", ".git/", "node_modules/"];
-
-	pi.on("tool_call", async (event, ctx) => {
-		if (event.toolName !== "write" && event.toolName !== "edit") {
-			return undefined;
-		}
-
-		const path = event.input.path as string;
-		const isProtected = protectedPaths.some((p) => path.includes(p));
-
-		if (isProtected) {
-			if (ctx.hasUI) {
-				ctx.ui.notify(`Blocked write to protected path: ${path}`, "warning");
-			}
-			return { block: true, reason: `Path "${path}" is protected` };
-		}
-
-		return undefined;
-	});
-}