Complete reviewed terminal blockers

[areshand]

Summary

• allow a reviewed external-only workflow to return a terminal structural blocker when no operation succeeded and no executor failure occurred
• keep failed-only executions fail-closed
• expose one typed, writable resultCandidate.path in workflow context and direct the orchestrator to use it instead of supervisor-owned workflow paths
• distinguish successful, failed, and blocked receipt counts in lifecycle evidence

Production UX evidence

The production PR-review regression reached a terminal github_repository_inaccessible receipt before any successful operation. The old external-only gate rejected the honest blocker because it required one success, then the orchestrator retried incompatible completion routes until interruption. The same run also attempted its result handoff inside the supervisor-owned workflow directory and received EACCES.

Security properties

• blocked-only completion still requires a terminal reviewed receipt and the existing external completion gate
• any executor failure without a success continues to reject completion
• supervisor-owned workflow permissions are unchanged
• the candidate result remains under MULTIAGENT_STATE_DIR and is validated and canonically persisted by the supervisor

Validation

• 72 Rust unit tests
• lifecycle integration tests, including success+blocked, blocked-only, and failed-only receipt cases
• full tests/run.sh framework, lifecycle, mock E2E, and boundary suite

[areshand]

Superseded by merged PR #74, which carries the reviewed terminal-blocker completion path onto the phase-one repository layout and passes the full current CI matrix.