Build the offline cross-runtime collaboration harness #200

Implementation slice 1 of #198, following the contract proposed in #199.

Build an offline-only, deterministic harness with distinct synthetic owner identities. Preserve same-owner friendship rejection. Add a versioned text codec and canonical input digests, scripted peers, a restartable journal and fake clock.

Acceptance:

• Demonstrate request, clarification, answer, result-ready and explicit requester acceptance; parallel tasks keep separate sessions.
• Cover scoped grants and independent approvals, malformed/conflicting messages, duplicates, out-of-order and late events, revocation, cancellation and ambiguous submission recovery.
• Persist work reservations and resource bounds through restart; missing real usage stays unknown.
• Run without credentials, network, a real OpenCode process or model calls; clearly distinguish mock semantics from real API compatibility.
• Wire the suite into local verification and required CI; document commands and remaining host/social/real-runtime gaps.

No production execution path, permission change, same-owner exception, A2A interoperability claim or team-mode enablement. Keep #198 open.