fix: invalidate caches before roadmap check in /gsd discuss (#1240)

After milestone creation, the file parse caches could still hold
stale results from before the ROADMAP was written. /gsd discuss checks
for ROADMAP but hits the cache, gets null, and reports 'No roadmap yet'
even though the file exists on disk.

Added invalidateAllCaches() at the top of showDiscuss() to ensure
fresh reads from disk.

Fixes #1236

src/resources/extensions/gsd/guided-flow.ts

@@ -415,6 +415,9 @@ export async function showDiscuss(
     return;
   }
 
+  // Invalidate caches to pick up artifacts written by a just-completed discuss/plan
+  invalidateAllCaches();
+
   const state = await deriveState(basePath);
 
   // Guard: no active milestone