feat: render thinking process UI for intermediate agent messages

Wire message-reclassified sidecar events through the main process to
the renderer, where reclassified messages are filtered out of the main
transcript and collected into a collapsible ThinkingProcess component.

Main process changes:
- Route message-reclassified via dedicated onMessageReclassified callback
- Add applyMessageReclassified handler that sets messageKind and emits
  the session event for the renderer
- Forward assistant-intent and reasoning-delta as turn-scoped events

Renderer changes:
- Split session.messages into visibleMessages and thinkingMessages
- Render ThinkingProcess above the last assistant message
- ThinkingProcess auto-expands during active turns, collapses on finish
- Update messagePhase to skip thinking-kind messages for final detection

Tests:
- 3 new sessionWorkspace tests for reclassification apply/dedup/ignore
- 2 new messagePhase tests for thinking-kind handling
- Updated runTurnPending test fixtures for new callback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/main/runTurnPending.test.ts

@@ -19,6 +19,7 @@ describe('run turn pending helpers', () => {
       onUserInput: () => undefined,
       onExitPlanMode: () => undefined,
       onMcpOAuthRequired: () => undefined,
+      onMessageReclassified: () => undefined,
       onTurnScopedEvent: () => undefined,
       errored: false,
     };
@@ -45,6 +46,7 @@ describe('run turn pending helpers', () => {
       onUserInput: () => undefined,
       onExitPlanMode: () => undefined,
       onMcpOAuthRequired: () => undefined,
+      onMessageReclassified: () => undefined,
       onTurnScopedEvent: () => undefined,
       errored: false,
     };