ci(relayflow): stage exact broker proof artifacts (#1632)

* ci(relayflow): stage exact broker proof artifacts

* fix(ci): trust and await exact broker artifacts

* fix(ci): validate broker polling bounds

* fix(ci): bind proof broker execution

* fix(ci): retain trusted broker artifacts

* fix(ci): cover broker build timeout

* fix(ci): compose proof deadlines

* fix(ci): block fork broker builds

* fix(ci): cancel duplicate broker builds

* fix(ci): seal proof broker executable

* fix(ci): bound broker artifact resolution

* fix(ci): isolate PR broker build cache

* fix(ci): harden broker proof deadlines

* fix(ci): preserve broker self-spawn in PR proofs

* fix(ci): bound concurrent broker resolution

* docs(trail): synthesize PR proof final state

* fix(ci): isolate PR-authored broker builds

* docs(trail): list trusted broker events

* docs(trail): mark cache design superseded

* fix(ci): fail closed on broker reap

* fix(ci): fail closed on builder probe errors

* fix(ci): prove builder process visibility

* docs(trail): mark probe design superseded

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Co-authored-by: Proactive Runtime Bot <agent@agent-relay.com>