feat(agent-interface): bind benchmark graders to execution plans

[drewstone]

Problem

Candidate execution plans did not identify the benchmark grader, so a later execution could score the same plan with a different implementation.

Change

Add a strict grader identity (name, version, content-addressed artifact) to the immutable execution-plan material and update fixtures.

Verification

• pnpm --filter @tangle-network/agent-interface check-types
• pnpm --filter @tangle-network/agent-interface build
• pnpm --filter @tangle-network/agent-interface test
• 10 test files, 104 tests passed

[tangletools]

✅ Auto-approved drewstone PR — 36de7f63

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-07-11T07:10:44Z}