Sandcastle: log skip-flag usage to audit trail and PR body

• SANDCASTLE_SKIP_GATES_VERIFY=1 — skips the gates-status.json verification (main.mts:602).
• SANDCASTLE_SKIP_SECRET_SCAN=1 — skips the secret scanner before push (main.mts:648).

Both emit only a console.warn on stdout. For local dev this is fine; as soon as the workflow runs in CI or a teammate exports them once in their shell rc, there's no audit trail tying a pushed branch back to which gate was waived.

Suggested action:

1. When either flag is set, append a "⚠️ skipped: <flag>" field to the PR body so reviewers see it before merging.
2. Append a line to .sandcastle/audit.log (one per run) recording {timestamp, branch, plan, skipped_flags} for after-the-fact forensics. Gitignore the file but keep it host-side.
3. Optional: refuse the skip flags entirely when CI=true.

File: .sandcastle/main.mts:602-606, 648-652