api(tasks): claim_task reassigns a pre-assigned task to any caller, overriding delegator intent #468

Summary

claim_task authorizes against the request body, not the stored row. The handler only checks the N13 body binding auth == body.assignee_did (crates/gitlawb-node/src/api/tasks.rs:175), then db.claim_task sets assignee_did=$2 on any pending row unconditionally (crates/gitlawb-node/src/db/mod.rs:3743-3757):

UPDATE agent_tasks SET status='claimed', assignee_did=$2, updated_at=$3
WHERE id=$1 AND status='pending'

complete_task/fail_task (tasks.rs:224, :277) then authorize against the stored assignee, so the new assignee of record passes them.

Impact

A signed caller other than the delegator's chosen assignee can take over a pending pre-assigned task and then legitimately complete or fail it. The schema (db/mod.rs:626-644) has no trigger or constraint preventing reassignment. Read-side exposure of the same objects is tracked in #268 and #395; this is the write-side ACL gap with a different root cause (N13 bound the caller to the body, never to the stored row).

Remediation

1. Extend the claim predicate: AND (assignee_did IS NULL OR assignee_did = $2).
2. Return 403 (or 409) when a pre-assigned pending task is claimed by another identity.
3. Integration test: a third-DID claim of a pre-assigned pending task must fail and not mutate the row.