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/test/session-manager/migration.test.ts

@@ -24,8 +24,8 @@ describe("migrateSessionEntries", () => {
 
 		migrateSessionEntries(entries);
 
-		// Header should have version set
-		expect((entries[0] as any).version).toBe(2);
+		// Header should have version set (v3 is current after hookMessage->custom migration)
+		expect((entries[0] as any).version).toBe(3);
 
 		// Entries should have id/parentId
 		const msg1 = entries[1] as any;

packages/coding-agent/test/session-manager/save-entry.test.ts

@@ -9,7 +9,7 @@ describe("SessionManager.saveCustomEntry", () => {
 		const msgId = session.appendMessage({ role: "user", content: "hello", timestamp: 1 });
 
 		// Save a custom entry
-		const customId = session.appendCustomEntry("my_hook", { foo: "bar" });
+		const customId = session.appendCustomEntry("my_data", { foo: "bar" });
 
 		// Save another message
 		const msg2Id = session.appendMessage({
@@ -36,7 +36,7 @@ describe("SessionManager.saveCustomEntry", () => {
 
 		const customEntry = entries.find((e) => e.type === "custom") as CustomEntry;
 		expect(customEntry).toBeDefined();
-		expect(customEntry.customType).toBe("my_hook");
+		expect(customEntry.customType).toBe("my_data");
 		expect(customEntry.data).toEqual({ foo: "bar" });
 		expect(customEntry.id).toBe(customId);
 		expect(customEntry.parentId).toBe(msgId);

packages/coding-agent/test/session-manager/tree-traversal.test.ts

@@ -89,7 +89,7 @@ describe("SessionManager append and tree traversal", () => {
 			const session = SessionManager.inMemory();
 
 			const msgId = session.appendMessage(userMsg("hello"));
-			const customId = session.appendCustomEntry("my_hook", { key: "value" });
+			const customId = session.appendCustomEntry("my_data", { key: "value" });
 			const _msg2Id = session.appendMessage(assistantMsg("response"));
 
 			const entries = session.getEntries();
@@ -97,7 +97,7 @@ describe("SessionManager append and tree traversal", () => {
 			expect(customEntry).toBeDefined();
 			expect(customEntry.id).toBe(customId);
 			expect(customEntry.parentId).toBe(msgId);
-			expect(customEntry.customType).toBe("my_hook");
+			expect(customEntry.customType).toBe("my_data");
 			expect(customEntry.data).toEqual({ key: "value" });
 
 			expect(entries[2].parentId).toBe(customId);