fix(tui): keep pending dialog when a nested sub-agent stream starts

[aheritier]

Summary

Fixes #3217.

A user_prompt/elicitation (or tool-confirmation / max-iterations) dialog could be silently dropped when a sub-agent or fork-skill stream started in the same session, leaving the run blocked on user input with no visible dialog.

Root cause

pkg/tui/service/supervisor/supervisor.go → handleRuntimeEvent cleared runner.PendingEvent (and toggled IsRunning/NeedsAttn) on any StreamStarted/StreamStopped event, regardless of which session it belonged to. transfer_task and fork-mode run_skill forward the child session's stream lifecycle events up through the parent's event channel; those nested events carry a different SessionID than the runner key, so they wiped the parent runner's pending attention event. runner.PendingEvent is the only thing that keeps a backgrounded attention dialog alive across a tab switch/re-render, so once cleared, ConsumePendingEvent returned nil, the dialog was never re-displayed, and the runtime stayed blocked in elicitationHandler.

Fix

Add an isTopLevelStream(runnerID, evSessionID string) bool helper (evSessionID == "" || evSessionID == runnerID) and gate both the StreamStartedEvent and StreamStoppedEvent state mutations on it. Nested sub-session streams no longer toggle IsRunning or clear the parent runner's pending event; top-level turns still supersede stale pending events exactly as before. An empty SessionID is treated as top-level for backward compatibility (mirrors the existing handleTokenUsage guard).

This also corrects a latent tab-indicator bug: a nested StreamStopped no longer flips the parent tab's IsRunning to false mid-turn.

Tests

Added to pkg/tui/service/supervisor/supervisor_test.go:

• TestIsTopLevelStream — table: matching / empty / child / sibling session IDs
• TestStreamStarted_SubSessionDoesNotDropPendingEvent
• TestStreamStopped_SubSessionDoesNotDropPendingEvent
• TestStreamStarted_TopLevelSupersedesStalePending
• TestStreamStopped_TopLevelClearsPendingAndNeedsAttn — covers all three attention event types (elicitation, tool confirmation, max-iterations)
• TestStreamStarted_EmptySessionID_TreatedAsTopLevel

Validation

• task build ✅
• task test ✅ (only pre-existing, unrelated baseline failures)
• go test -race ./pkg/tui/service/supervisor/... ✅
• task lint ✅ (only pre-existing SlogContextual warnings)

[docker-agent]

Assessment: 🟢 APPROVE

The fix is correct and well-targeted. isTopLevelStream cleanly guards both StreamStartedEvent and StreamStoppedEvent handlers so that nested sub-session stream lifecycle events (carrying a child SessionID) no longer wipe the parent runner's PendingEvent or toggle IsRunning. The empty-SessionID backward-compatibility path matches the existing convention in handleTokenUsage and is clearly documented. The six new tests cover the key cases — sub-session no-ops, top-level superseding stale events, and all three attention event types on StreamStopped.

No bugs were found in the introduced changes.