test(governance): disposable writer canary A

[100yenadmin]

Disposable content-free canary for accepted issue #357. This PR will not merge. It exists only to verify exact-head check serialization, invalid-receipt blocking, and base-retarget reset behavior. No product, release, runtime, or customer scope.

[100yenadmin]

Closing — disposable canary, purpose served, evidence preserved in #361.

Frozen head: fedbac6.

What this canary proved (recorded in #361's Durable Plan Contract): merged PR #358 fixed the
stale-success writer race, but unconditional global failure reconciliation allows only ONE
independently valid open PR to remain green at a time. That is the defect #361 owns and PR #362
fixes; fresh two-PR, invalid-receipt, head-change and base-retarget canaries will be run against
#362 rather than resurrected here.

Terminal receipt per #361's done-when ("#357/#359/#360 receive terminal receipts"). No product,
runtime, or ruleset delta occurred on this branch.