feat(continual-learning): derive wasRealCollision from git overlap (RSI step 3)

Backend becomes authoritative for the verifier signal. recordOutcome now
overrides the client-supplied wasRealCollision with deriveWasRealCollision():
a flagged collision counts as REAL only if BOTH named engineers currently have
the collided file in their git changedFiles (getGitStates, 120s freshness TTL).
Conservative false when the collision is orphaned/missing or git state is
stale. Restores the (accepted x wasReal) 2x2 the spec assumes instead of the
hardcoded 107/107 true.

frontend/useInterventions.ts stops sending a hardcoded `true` (now a
backend-overridden placeholder). Spec: continual-learning/spec.md:98-108,
policy.md:35-42. backend+frontend typecheck + eslint pass. PLAN.md P0.5 rung 3
added.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/livekit/useInterventions.ts

@@ -66,7 +66,9 @@ export function useInterventions(room: Room | null) {
         interventionId: active.id,
         collisionId: active.collisionId,
         podId: active.podId,
-        wasRealCollision: true,
+        // Placeholder only — the backend derives the authoritative value from
+        // git overlap at outcome time (the client cannot know). (RSI Step 3)
+        wasRealCollision: false,
         accepted,
         recordedAt: new Date().toISOString(),
       });