XY-701: Lane control P0: specify CLI/API-first capabilities and update Decodex skills

[yvette-carlisle]

Summary:

• Add docs/spec/lane-control.md with the supported/deferred lane-control capability matrix.
• Update app-server and operator-control docs for CLI/API-first steer and interrupt semantics, deferred raw thread injection, and UI posture.
• Update Decodex skills so agents inspect first, prefer soft interrupt, use operator-supplied steer only through CLI/API, and treat task replacement as lifecycle work.

Verification:

• semantic drift audit: pass
• cargo make fmt
• cargo make lint-fix
• cargo make test (919 passed, 1 skipped)