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/src/modes/rpc/rpc-client.ts

@@ -196,7 +196,7 @@ export class RpcClient {
 	/**
 	 * Start a new session, optionally with parent tracking.
 	 * @param parentSession - Optional parent session path for lineage tracking
-	 * @returns Object with `cancelled: true` if a hook cancelled the new session
+	 * @returns Object with `cancelled: true` if an extension cancelled the new session
 	 */
 	async newSession(parentSession?: string): Promise<{ cancelled: boolean }> {
 		const response = await this.send({ type: "new_session", parentSession });
@@ -330,7 +330,7 @@ export class RpcClient {
 
 	/**
 	 * Switch to a different session file.
-	 * @returns Object with `cancelled: true` if a hook cancelled the switch
+	 * @returns Object with `cancelled: true` if an extension cancelled the switch
 	 */
 	async switchSession(sessionPath: string): Promise<{ cancelled: boolean }> {
 		const response = await this.send({ type: "switch_session", sessionPath });
@@ -339,7 +339,7 @@ export class RpcClient {
 
 	/**
 	 * Branch from a specific message.
-	 * @returns Object with `text` (the message text) and `cancelled` (if hook cancelled)
+	 * @returns Object with `text` (the message text) and `cancelled` (if extension cancelled)
 	 */
 	async branch(entryId: string): Promise<{ text: string; cancelled: boolean }> {
 		const response = await this.send({ type: "branch", entryId });