fix(pilot): wipe full task-lane state on chat switch + extract palette event

Two fixes from the Phase 5 shakedown:

1. Stale lane data leaking across chats. handleNewChat, sendPrefill, and
   handleResumeNew were each missed when Phase 3/5 added activeFix,
   previewKind, previewData, and scriptPanelOpen — only selectChat reset
   the full set. Result: starting a new chat while a Suggested Fix card
   was active showed the previous session's fix card (and any open
   preview/script panel) until the next backend refresh swept it.
   Consolidated all four entry points behind a single
   resetSessionDerivedState() helper so adding new lane state in future
   phases only requires touching one place.

2. CommandPalette TDZ on cold load. SCRIPTS_INLINE_QUICK_ACTION (line 66)
   referenced PILOT_INLINE_SCRIPT_PATH declared at line 94 — module-level
   evaluation hit the use before the declaration. Browser blanked with
   "Cannot access 'PILOT_INLINE_SCRIPT_PATH' before initialization".
   Moved the path const above its first use; also extracted
   PILOT_INLINE_SCRIPT_EVENT into a tiny @/lib/pilotEvents module so
   AssistantChatPage doesn't import the palette component just to read a
   string — that mixed-export pattern broke Fast Refresh ("consistent
   components exports") and added an unnecessary import edge.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/pilotEvents.ts

@@ -0,0 +1,14 @@
+/**
+ * Cross-component event names for the FlowPilot session UI.
+ *
+ * Lives in /lib (not /components) so importing the event name does NOT
+ * pull in any React component module. AssistantChatPage and CommandPalette
+ * both reference it without forming an import cycle, and Vite's
+ * react-fast-refresh "consistent components exports" check stays happy.
+ */
+
+// Phase 5: dispatched by the global Cmd+K palette when the engineer picks
+// "Open inline Script Generator" while on a /pilot/:id route. The chat
+// page subscribes via `window.addEventListener` and toggles its inline
+// Script Generator panel if there's an active suggested fix.
+export const PILOT_INLINE_SCRIPT_EVENT = 'flowpilot:open-inline-script'