Canary QA worker role

[ce-overwatch]

Summary

• Defines canary_qa as a disposable-scratch released-artifact validation role with live-artifact egress and sandbox-repository short-TTL credentials only.
• Updates the worker role roster (.claude/agents/README.md) so controllers can dispatch the new role.
• Supersedes brain-assertion-d1b-20-canonical-worker-role-files-v2 → -v3: names canary_qa as the fifth canonical role and re-pins evidence_sha256 to this branch's own .claude/agents/README.md, preserving the ad-hoc-role-invention guard clause — this supersede is the deliberate, controller-authorized broadening it warns against, not an ad hoc addition.

Test plan

• ce brain verify --drift --state-root .ce — OK (138 records, 40 active)
• ce brain verify --state-root .ce — OK
• Full ce validate-pr --declared-work-class tiny GREEN one pass (only known pre-existing aarch64 test_install_sh_uv_hash_mismatch_... baseline failure; zero new failures)

Closes creator-engine/ce-ops#452

• Declared work class: tiny

ce-approval-capability: v1.eyJhcHByb3ZlZF9ieSI6ImNlLWRldi0yIiwiZXhwaXJlc19hdCI6MTc4MzI4MzE0MywiaGVhZF9zaGEiOiJmYjYyMzk0MjUxNmExZTk5ZDI1ODE5NmNkOTE2ZDE3ZmQ3MzY3ZTY5IiwiaXNzdWVkX2F0IjoxNzgzMjc5NTQzLCJwb2xpY3lfc2hhIjoiNzliOWRjOGI0MjllNGFmMTA5ZWNjNjhhZjE5YjI2ZTliM2Y2NDdkOWZhZDhkZDM3MzA5ZDZmNWVhZDgxYjNiMCIsInByX251bWJlciI6ODM2LCJyZXBvIjoiY3JlYXRvci1lbmdpbmUvY3JlYXRvci1lbmdpbmUifQ.YF6uhCYItZVbJOW6l-bdhdSGNEVrPHClzu_UGHnnN3k

[ce-dev-2]

Banked content APPROVE, submitted after post-#835 recompute verification at fb62394. Original review verified the canary_qa role contract: tools coherent with the disposable-scratch-only mount (Read/Grep/Glob/Bash, no write tools), explicit credential exclusions (no SSH/host/browser/controller-key material), signature-invalid=STOP-and-route-to-controller line covering the motivating incident class, and the d1b-20 supersede chain fields verified structurally. Post-recompute delta verified by controller: effective diff vs main = exactly the 6 carrier paths; role file contract lines intact; d1b-20 v2→v3 correctly chained from d1b-19-v3's content_hash 8dc901e0… (sequence 139→141), active-count 91 grep-verified, evidence pin (README.md sha 95b69543…) unchanged, ce brain verify OK (142 records), validate-pr GREEN vs post-#835 base.