fix(run): prevent premature exit on idle before meaningful work (#1263)

The run command's completion check had a race condition: when a session
transitions busy->idle before the LLM generates any output (empty
response or API delay), checkCompletionConditions() returns true because
0 incomplete todos + 0 busy children = complete. This caused the runner
to exit with 'All tasks completed' before any work was done.

Fix:
- Add hasReceivedMeaningfulWork flag to EventState
- Set flag on: assistant text content, tool execution, or message update
  with actual content (all scoped to main session only)
- Guard completion check in runner poll loop: skip if no meaningful work
  has been observed yet

This ensures the runner waits until the session has produced at least one
observable output before considering completion conditions.

Adds 6 new test cases covering the race condition scenarios.

src/cli/run/runner.ts

@@ -143,6 +143,14 @@ export async function run(options: RunOptions): Promise<number> {
           process.exit(1)
         }
 
+        // Guard against premature completion: don't check completion until the
+        // session has produced meaningful work (text output, tool call, or tool result).
+        // Without this, a session that goes busy->idle before the LLM responds
+        // would exit immediately because 0 todos + 0 children = "complete".
+        if (!eventState.hasReceivedMeaningfulWork) {
+          continue
+        }
+
         const shouldExit = await checkCompletionConditions(ctx)
         if (shouldExit) {
           console.log(pc.green("\n\nAll tasks completed."))