feat(workspaces): encrypted write-only credential storage

Workspaces Payload collection with the PAT as a write-only field:
access.read () => false strips it from every access-controlled read
path, and a field-level beforeChange hook encrypts on write with
AES-256-GCM (scrypt key from VEECT_SECRET, random per-value salt + IV,
v1 storage format) via node:crypto only. The real repository replaces
the phase-1 stub with payload create/findByID; toDomain never maps the
credential, and getDecryptedCredential(id) is the single server-side
decrypt path for the runner handoff (story 07). Contract suite now
covers create, write-only behaviour, and the decrypt path against both
the mock and the Payload impl (stub runs the real collection hooks).
Missing VEECT_SECRET fails production bind/boot with an actionable
message; dev-seed boots without it. Env declared in turbo.json
globalEnv + .env.example.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

turbo.json

@@ -19,7 +19,8 @@
     "VERCEL_GIT_COMMIT_SHA",
     "NEXT_PUBLIC_VERCEL_GIT_COMMIT_SHA",
     "VERCEL_ENV",
-    "AUDIT_PSEUDONYM_SALT"
+    "AUDIT_PSEUDONYM_SALT",
+    "VEECT_SECRET"
   ],
   "boundaries": {
     "tags": {