Framework provider leases: ensure only the repo runtime can own or talk to a provider session

[FidoCanCode]

Parent: #694
Framework umbrella: #681

Problem

Today too many paths can end up talking to the provider or holding session-related coordination state. The framework needs one lease/ownership boundary so only the repo runtime may drive provider turns, model changes, cancels, and result consumption.

Deliverables

• a provider lease API owned by the repo runtime
• explicit lease transfer rules for active work, parked work, and preemption
• prohibition on direct provider/session use outside runtime actions
• integration path for direct provider child processes under runtime ownership

Done when

• every provider turn is owned by exactly one repo-runtime path
• no webhook/worker/helper code can talk to the provider without going through the lease API
• lease transfer during real preemption is explicit instead of implicit timing behavior
• provider misuse becomes a type/API violation instead of a timing bug

[FidoCanCode]

Superseded by D4 (session ownership FIFO) under the proof lane #710.

Under the unified plan on #396, coordination primitives are implemented via Rocq extraction (Band D in #710) instead of hand-written Python. The work intended by this issue is done in the referenced D-item(s), which carries the proof of correctness alongside the implementation.

Closing as superseded — not as completed — so the history is preserved. Subscribe to #710's children for progress on the replacement.