docs: define local workbench-agent boundary

[frahlg]

Summary

Docs-only direction checkpoint for Hugin Agent. The agent may later be Hugin's local hardware execution plane, but Device Support remains the canonical driver/package authority and FTW/Blixt remain the production runtimes.

Recorded TODOs

• Read-only probing and draft execution by default, enforced per host call.
• Explicit instruction/time/memory/network/response budgets and device allowlists.
• Structured evidence bound to exact source/package hash, target profile and hardware identity.
• Separate FTW GopherLua and Blixt LuaJIT test lanes; no inferred compatibility.
• Signed Device Support package verification for workbench reference only, never production install/activation.
• Reconciled local/NATS pairing, credential persistence, rotation, revocation and offline semantics.
• Any future control test requires physical confirmation, a short lease, default-mode recovery and structured outcomes.
• SDM630 telemetry-only first; Zap later.

Non-goals

No endpoint, sandbox, protocol, pairing, NATS, update or runtime behavior changes in this PR.

Draft for security/runtime review.