capability: bind a confirmation phrase to its subject #404

Kind: capability

Epic: #380

Part

The confirmation phrase satisfies only the waiting gate whose subject it names.
When the goal title or body changes, that phrase is stale and does not carry.
Filing the phrase onto the record does not change the subject it names.

Depends on

phrase-catalog. Also #372, because saving the agreement must not invalidate it.

Acceptance boundary

Change class: high-assurance.

The matching phrase answers that gate. A changed title or body leaves the old phrase stale. Saving the phrase still matches.

Excludes

Rebinding the phrase onto another subject. Deciding whether every comment or label also stales a revision.

Part of #380

Replaces #382