feat(delegation): add cancel-ack + force-abort frames and cancel lifecycle states

[hishamkaram]

What

Wire protocol surface for reliable delegation cancel/teardown (producer of a 3-repo feature).

• MsgDelegationCancelAck + DelegationCancelAckPayload — receipt confirmation for a delegation_cancel (ack→terminal split, modeled on the in-repo GitSyncCancelResponse precedent): Accepted=true means teardown started (terminal delegation_result follows), Accepted=false + redacted Error means it could not start.
• MsgDelegationForceAbort + DelegationForceAbortPayload — escalation of a stalled cancel.
• Intermediate states DelegationStateCancelling / CancelStalled / ForceKilled carried in DelegationStatusPayload.State for transient UI. The canonical terminal frame stays delegation_result{status:"cancelled"} so an old PWA self-heals on the result frame.
• Optional request_id idempotency key on DelegationCancelPayload.

Safety

All new fields omitempty / byte-additive (a pre-feature producer marshals byte-identical JSON). The ack/force-abort frames are pure control-plane signals: a no-content-on-wire test pins their key set to session IDs + request_id + accepted + redacted error only — never delegate output, inherited context, or diffs.

Tests

Round-trip + omitempty + lifecycle-state-constant + message-type-constant + no-content-on-wire tests. go vet, go build, go test -race -count=1 -p 4 (coverage 88.6% ≥ 83%), golangci-lint 0 issues — all green.

Producer-first

This is the producer in a cross-repo feature; it merges + tags FIRST, then agentd re-pins its go.mod and agentd + agentd-web follow.