Preserve informative terminal outcomes and blockers

[areshand]

Summary

• preserve bounded final agent messages across complete and interrupted worker outcomes
• reconcile stale worker addresses once before reporting subagent status unavailable
• expose a supervisor-validated result candidate path and allow honest reviewed structural blockers to terminate external-only work
• update the architecture decision record for the affected trust-boundary behavior

This consolidates and supersedes the useful changes from #68 and #72 on top of the phase-one repository layout.

Validation

• cargo fmt --all --check
• cargo test --locked --package multiagent (73 passed)
• npm test --prefix client (21 passed)
• npm test --prefix control-server (47 passed)
• python3 -m unittest discover -s tests -p test_*.py -v (93 passed)
• bash tests/run.sh