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/core/index.ts

@@ -13,26 +13,49 @@ export {
 } from "./agent-session.js";
 export { type BashExecutorOptions, type BashResult, executeBash } from "./bash-executor.js";
 export type { CompactionResult } from "./compaction/index.js";
-export {
-	type CustomTool,
-	type CustomToolAPI,
-	type CustomToolFactory,
-	type CustomToolsLoadResult,
-	type CustomToolUIContext,
-	discoverAndLoadCustomTools,
-	type ExecResult,
-	type LoadedCustomTool,
-	loadCustomTools,
-	type RenderResultOptions,
-} from "./custom-tools/index.js";
 export { createEventBus, type EventBus, type EventBusController } from "./event-bus.js";
+
+// Extensions system
 export {
-	type HookAPI,
-	type HookContext,
-	type HookError,
-	type HookEvent,
-	type HookFactory,
-	HookRunner,
-	type HookUIContext,
-	loadHooks,
-} from "./hooks/index.js";
+	type AgentEndEvent,
+	type AgentStartEvent,
+	type AgentToolResult,
+	type AgentToolUpdateCallback,
+	type BeforeAgentStartEvent,
+	type ContextEvent,
+	discoverAndLoadExtensions,
+	type ExecOptions,
+	type ExecResult,
+	type ExtensionAPI,
+	type ExtensionCommandContext,
+	type ExtensionContext,
+	type ExtensionError,
+	type ExtensionEvent,
+	type ExtensionFactory,
+	type ExtensionFlag,
+	type ExtensionHandler,
+	ExtensionRunner,
+	type ExtensionShortcut,
+	type ExtensionUIContext,
+	type LoadExtensionsResult,
+	type LoadedExtension,
+	type MessageRenderer,
+	type RegisteredCommand,
+	type SessionBeforeBranchEvent,
+	type SessionBeforeCompactEvent,
+	type SessionBeforeSwitchEvent,
+	type SessionBeforeTreeEvent,
+	type SessionBranchEvent,
+	type SessionCompactEvent,
+	type SessionShutdownEvent,
+	type SessionStartEvent,
+	type SessionSwitchEvent,
+	type SessionTreeEvent,
+	type ToolCallEvent,
+	type ToolDefinition,
+	type ToolRenderResultOptions,
+	type ToolResultEvent,
+	type TurnEndEvent,
+	type TurnStartEvent,
+	wrapToolsWithExtensions,
+} from "./extensions/index.js";