feat(exact-process): define retained process recovery

[drewstone]

What

• Define a retained, keyed exact-process contract in agent-interface.
• Make the provider testkit prove launch idempotency, filtered lookup, and fresh-worker recovery.

Boundary

This is only the shared contract and testkit. The Sandbox adapter and Runtime consumer wait for a published Sandbox version; no local links or compatibility shims.

Proof

• @tangle-network/agent-interface check-types
• @tangle-network/agent-provider-testkit test (2/2)
• @tangle-network/agent-provider-testkit check-types
• Clean merge tree against origin/main.

[drewstone]

Closing this draft because CI proved the split is invalid: the shared contract now requires keyed recovery, while the Tangle adapter has not yet adopted Sandbox 0.12. The correct order is Sandbox 0.12, then one agent-sdk PR containing both the contract and adapter, then Runtime. The branch remains as a reusable candidate; the contract will not be weakened.