Supply-chain audit: cover packages/agent-sdk/constraints.txt with pip-audit

`tools/check_supply_chain.py audit` runs pip-audit only on `requirements*.txt`, so the Agent SDK's hash-pinned `packages/agent-sdk/constraints.txt` (the exact set `check.sh` installs) is never checked for known vulnerabilities, and its pins move only by hand. Add it to the weekly `audit` (`pip-audit -r constraints.txt --require-hashes`), and decide whether dependabot or a documented bump should keep its pins and hashes current. Out of scope: the runner CLI locks (already gated), the SDK's own version ranges, publishing the SDK. Done means: - `python tools/check_supply_chain.py audit` names `packages/agent-sdk/constraints.txt` among the scanned sets, and a HIGH advisory planted in a copy fails it. Required checks: ./dc check full

The weekly audit now pip-audits packages/agent-sdk/constraints.txt with --require-hashes and lists it; a planted HIGH fails it; the SDK pyproject files count as audited through it. Pins stay a documented hand bump (Dependabot can't read the file). Quick gate green; PR #6027 queued.