fix: remove stale port 5173 references from board mutation guard

Update dev origin allowlist and tests to use port 3100 only, matching
the unified dev server setup. Fix AGENTS.md and README accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -40,7 +40,7 @@ pnpm dev
 This starts:
 
 - API: `http://localhost:3100`
-- UI: `http://localhost:5173`
+- UI: `http://localhost:3100` (served by API server in dev middleware mode)
 
 Quick checks:
 
@@ -140,4 +140,3 @@ A change is done when all are true:
 2. Typecheck, tests, and build pass
 3. Contracts are synced across db/shared/server/ui
 4. Docs updated when behavior or commands change
-