feat(interface): add exact retained cancellation

[drewstone]

What changed

• add an all-or-nothing retained-run capability declaration
• add digest-bound cancellation requests and exact acknowledgements
• expose retry-safe run cancellation on AgentSession

Proof

• pnpm --filter @tangle-network/agent-interface check-types
• pnpm --filter @tangle-network/agent-interface test — 332/332
• pnpm --filter @tangle-network/agent-interface build
• node scripts/check-control-contract-artifacts.mjs — 118/118 from packed artifacts
• independent adversarial review completed; provider/server findings are being fixed separately before their release