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/hello/index.ts

@@ -0,0 +1,21 @@
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import { Type } from "@sinclair/typebox";
+
+export default function (pi: ExtensionAPI) {
+	pi.registerTool({
+		name: "hello",
+		label: "Hello",
+		description: "A simple greeting tool",
+		parameters: Type.Object({
+			name: Type.String({ description: "Name to greet" }),
+		}),
+
+		async execute(_toolCallId, params, _onUpdate, _ctx, _signal) {
+			const { name } = params as { name: string };
+			return {
+				content: [{ type: "text", text: `Hello, ${name}!` }],
+				details: { greeted: name },
+			};
+		},
+	});
+}