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/README.md

@@ -11,7 +11,7 @@ Programmatic usage of pi-coding-agent via `createAgentSession()`.
 | `03-custom-prompt.ts` | Replace or modify system prompt |
 | `04-skills.ts` | Discover, filter, or replace skills |
 | `05-tools.ts` | Built-in tools, custom tools |
-| `06-hooks.ts` | Logging, blocking, result modification |
+| `06-extensions.ts` | Logging, blocking, result modification |
 | `07-context-files.ts` | AGENTS.md context files |
 | `08-slash-commands.ts` | File-based slash commands |
 | `09-api-keys-and-oauth.ts` | API key resolution, OAuth config |
@@ -36,7 +36,7 @@ import {
   discoverAuthStorage,
   discoverModels,
   discoverSkills,
-  discoverHooks,
+  discoverExtensions,
   discoverCustomTools,
   discoverContextFiles,
   discoverSlashCommands,
@@ -89,7 +89,7 @@ const { session } = await createAgentSession({
   systemPrompt: "You are helpful.",
   tools: [readTool, bashTool],
   customTools: [{ tool: myTool }],
-  hooks: [{ factory: myHook }],
+  extensions: [{ factory: myExtension }],
   skills: [],
   contextFiles: [],
   slashCommands: [],
@@ -119,8 +119,8 @@ await session.prompt("Hello");
 | `tools` | `codingTools` | Built-in tools |
 | `customTools` | Discovered | Replaces discovery |
 | `additionalCustomToolPaths` | `[]` | Merge with discovery |
-| `hooks` | Discovered | Replaces discovery |
-| `additionalHookPaths` | `[]` | Merge with discovery |
+| `extensions` | Discovered | Replaces discovery |
+| `additionalExtensionPaths` | `[]` | Merge with discovery |
 | `skills` | Discovered | Skills for prompt |
 | `contextFiles` | Discovered | AGENTS.md files |
 | `slashCommands` | Discovered | File commands |