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/sdk/08-prompt-templates.ts

@@ -0,0 +1,42 @@
+/**
+ * Prompt Templates
+ *
+ * File-based templates that inject content when invoked with /templatename.
+ */
+
+import {
+	createAgentSession,
+	discoverPromptTemplates,
+	type PromptTemplate,
+	SessionManager,
+} from "@mariozechner/pi-coding-agent";
+
+// Discover templates from cwd/.pi/prompts/ and ~/.pi/agent/prompts/
+const discovered = discoverPromptTemplates();
+console.log("Discovered prompt templates:");
+for (const template of discovered) {
+	console.log(`  /${template.name}: ${template.description}`);
+}
+
+// Define custom templates
+const deployTemplate: PromptTemplate = {
+	name: "deploy",
+	description: "Deploy the application",
+	source: "(custom)",
+	content: `# Deploy Instructions
+
+1. Build: npm run build
+2. Test: npm test
+3. Deploy: npm run deploy`,
+};
+
+// Use discovered + custom templates
+await createAgentSession({
+	promptTemplates: [...discovered, deployTemplate],
+	sessionManager: SessionManager.inMemory(),
+});
+
+console.log(`Session created with ${discovered.length + 1} prompt templates`);
+
+// Disable prompt templates:
+// promptTemplates: []