Every mutating work-session route keyed off session_id alone after the
role gate, so any developer could commit into / abandon / complete a
peer's active session (breaking the single-active-WorkSession invariant
and stranding that task) and any PM could merge any cell's PR — the REST
surface bypassed the verb layer's active-claimant gate entirely. Add a
shared _assert_ownership guard: dev ops require session.agent_id to be
the caller; PM merge_pr requires a cell PM to own the session's task cell
(main PM / CEO / board coordinate every cell), 404 for a missing session.
merge_pr took merged_by from the request body, so any PM could record a
PR merge under another agent's id, corrupting the merge audit trail the
completion/CEO-approval chain and metrics rely on. Drop the body param
and stamp the authenticated caller's agent_id as merged_by (the
MergePRRequest schema is gone with it).
Tests: a second dev's token hitting a peer's /commits and /abandon -> 403
(session left active); a foreign-cell PM -> 403, same-cell PM -> 200; a
spoofed body merged_by is ignored and the persisted row records the PM.
Renn F
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