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/args.test.ts

@@ -133,15 +133,20 @@ describe("parseArgs", () => {
 		});
 	});
 
-	describe("--hook flag", () => {
-		test("parses single --hook", () => {
-			const result = parseArgs(["--hook", "./my-hook.ts"]);
-			expect(result.hooks).toEqual(["./my-hook.ts"]);
+	describe("--extension flag", () => {
+		test("parses single --extension", () => {
+			const result = parseArgs(["--extension", "./my-extension.ts"]);
+			expect(result.extensions).toEqual(["./my-extension.ts"]);
 		});
 
-		test("parses multiple --hook flags", () => {
-			const result = parseArgs(["--hook", "./hook1.ts", "--hook", "./hook2.ts"]);
-			expect(result.hooks).toEqual(["./hook1.ts", "./hook2.ts"]);
+		test("parses -e shorthand", () => {
+			const result = parseArgs(["-e", "./my-extension.ts"]);
+			expect(result.extensions).toEqual(["./my-extension.ts"]);
+		});
+
+		test("parses multiple --extension flags", () => {
+			const result = parseArgs(["--extension", "./ext1.ts", "-e", "./ext2.ts"]);
+			expect(result.extensions).toEqual(["./ext1.ts", "./ext2.ts"]);
 		});
 	});