feat(coder): add plugin hook host

- Typed hook registry with registerHook/emitHook/clearHooks
- Hooks: tool.execute.before/after, turn.start/end, task.terminal
- SUL patterns only (oh-my-openagent: architecture study, no code copy)

apps/coder/src/plugins/host.ts

@@ -0,0 +1,42 @@
+export type HookName =
+  | 'tool.execute.before'
+  | 'tool.execute.after'
+  | 'turn.start'
+  | 'turn.end'
+  | 'task.terminal';
+
+export interface ToolHookContext {
+  tool: string;
+  args: Record<string, unknown>;
+  projectRoot: string;
+  sessionId: string;
+}
+
+export interface ToolResultContext extends ToolHookContext {
+  result: unknown;
+}
+
+export type PluginHook = (ctx: any) => Promise<any>;
+
+const hooks = new Map<HookName, PluginHook[]>();
+
+export function registerHook(name: HookName, fn: PluginHook): void {
+  const list = hooks.get(name) || [];
+  list.push(fn);
+  hooks.set(name, list);
+}
+
+export async function emitHook(name: HookName, ctx: any): Promise<any> {
+  const list = hooks.get(name);
+  if (!list) return ctx;
+  let current = ctx;
+  for (const fn of list) {
+    const result = await fn(current);
+    if (result !== undefined) current = result;
+  }
+  return current;
+}
+
+export function clearHooks(): void {
+  hooks.clear();
+}