[nestorcolt]

Closes #9612

What changes

• Size writer (scripts.tick.steps.tasks._write_task): sends the fields it expects (difficulty: "") as the task_update if_match guard, so the ledger server compares and sets in one apply. A guarded silent skip is detected by comparing the returned task with the written values and raises RevisionConflict.
• Group release (_release_group): always sends the members it read as the task_ungroup if_match guard (empty list when none were expected).
• Ledger task_ungroup (scripts.swarm_ledger.ledger_groups): accepts if_match holding only group_members (a list of task ids) and refuses the release when the lead's members moved. Schema field added; LedgerClient.ungroup_tasks forwards the guard.
• Tick characterisation golden records: the 42 recorded size writes now carry if_match: {difficulty: ""}.
• Mutation survivors on _write_task: a two field silent skip test pins the joined field list; the cast type argument mutant is cleared (runtime no-op).
• Changelog fragment; type baseline lowered for ledger_client (433 to 428) and ledger_groups (119 to 92).

Evidence

• Red: Proofs run 41758087010 on the red test commit failed the planted concurrent size and regroup tests (DID NOT RAISE RevisionConflict; ungroup refused if_match). https://github.com/the-cloud-clockwork/agentihooks/actions/runs/41758087010
• Green before restack: Proofs run 41759125618, 233 passed. https://github.com/the-cloud-clockwork/agentihooks/actions/runs/41759125618
• Restacked onto dev after Let the task update guard match size and group members #9610 merged; Branch workflow lanes green on every pushed head; mutation preflight 41778748933 red on two _write_task survivors, fixed in 570295f; mutation preflight 41788429568 on that head green. https://github.com/the-cloud-clockwork/agentihooks/actions/runs/41788429568
• Review: Spec reader closed in round 2 (release always guarded, left out field read as empty, test added); Standards reader closed in round 3 (rename kept to changed, new refusal assertions use exact equality).

🤖 Generated with Claude Code

Parent intent

Project: the ledger, quota routing and herdr: durable Claude and Codex messaging, seats, intent, work beyond code, swarm health.

Phase :

Task t2374, Tick size writer and group release compare and set through the server guard: Master spec from an engineer follow up. Parent: tick writes to task size and grouping never overwrite a concurrent change. Seam: the tick size writer and the group release use the new ledger server guard, so both compare and set on the server, and the ungroup operation honours the same guard. Red then green in CI with a planted concurrent write. Done when merged.