feat(backends): resolve external CLI descriptors in the runtime

[elsheppo]

Summary

NEEDLE already loads external CLI backend descriptors, but runtime configuration still resolves only built-in backends. This change carries an explicitly selected descriptor and verified executable through normal config resolution and store construction, then uses the descriptor's atomic claim contract.

• resolve explicitly named external descriptors through the normal workspace config path
• preserve one verified binding through worker, supervisor, validation, doctor, and cross-workspace lookup
• verify executable identity before opening or mutating the store
• distinguish claimed, race-lost, not-claimable, malformed-response, and store-error outcomes
• preserve existing native backend behavior and restart-required configuration semantics
• document the trust boundary and provide a credential-free local fixture

Remote lease renewal and dispatch admission/finalization are intentionally separate follow-ups.

Validation

• cargo test --locked --test external_backend_runtime — 8 passed
• scripts/definition-of-done.sh --fast — 5 checks passed
• manual all-target test compilation on the complete downstream stack — passed

The repository's --all wrapper could not complete on the macOS validation host because it invokes GNU timeout, which is not installed there. A manual library run was compared with an untouched-base run and did not worsen the aggregate pre-existing failure count. I am not claiming the full suite is green; upstream CI remains authoritative.

Downstream proof

The complete three-layer contribution stack was deployed to a real external work-plane integration before submission. A 50-item production batch completed with 50 verified receipt references and no queued, running, or failed work remaining. Controlled restart recovery, expired-lease reclamation, duplicate/idempotent completion, expired-grant denial, invalid thread-correlation denial, credential isolation, app-server reconnect, and a full empty-queue backoff with zero launches all passed.

That downstream proof includes the separate lease-aware and dispatch-lifecycle follow-up patches; it is evidence that this extension point works in the intended integration, not a claim that this PR alone provides those later capabilities.