feat: add host-driven GitHub Actions CI watcher (Level 3)

Replace LLM-per-tick polling with direct `gh api` calls for GitHub
Actions watchers. New `ci_provider` discriminator column routes tasks
to either the host-driven path (zero LLM tokens) or the existing
generic LLM path. GitHub watchers poll at 15s intervals (min 10s)
via `checkGitHubActionsRun()` in the scheduler, with completion
messages sent directly to chat on terminal state.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/task-watch-status.ts

@@ -16,6 +16,12 @@ export function isWatchCiTask(task: Pick<ScheduledTask, 'prompt'>): boolean {
   return task.prompt.startsWith(WATCH_CI_PREFIX);
 }
 
+export function isGitHubCiTask(
+  task: Pick<ScheduledTask, 'ci_provider'>,
+): boolean {
+  return task.ci_provider === 'github';
+}
+
 export function isTaskStatusControlMessage(content: string): boolean {
   return content.startsWith(TASK_STATUS_MESSAGE_PREFIX);
 }