Crow personas and generated banter

- personas.go: backstory, prompt policy and output policy per crow;
  Hugin and Munin grounded in Grimnismal 20 and Gylfaginning 38;
  every working crow also follows a common output policy
- Personas go into each crow's instructions; Sentry keeps JSON-only
- banter.go: retrieve backstory, relationships and recent flock
  memory, then the speaking crow generates one line in character;
  max 2 per list against the list budget, never blocks work,
  unusable lines dropped
- crow.banter event, GET /api/crows, `crows personas` -> PERSONAS.md
- UI: epithets on tiles and messages, banter as quiet asides

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UQnSw4RRWp8uqpj3nsnnXF

brain.go

@@ -231,6 +231,8 @@ func (MockBrain) Ask(ctx context.Context, system, prompt string) (string, int, e
 	reportModel(ctx, "mock")
 	tokens := (len(system)+len(prompt))/4 + 40
 	switch {
+	case strings.Contains(system, markerBanter):
+		return "[mock] Caw.", tokens, nil
 	case strings.Contains(system, markerSentry):
 		return mockTriage(after(prompt, "ITEM:")), tokens, nil
 	case strings.Contains(system, markerElder):