refactor(contract): remove knowledge candidate schema versions

[drewstone]

Problem

Knowledge persisted a second version coordinate inside greenfield candidate and transaction records, while the shared interface now has one strict candidate shape.

Change

• remove candidate, evidence, run-state, and transaction schema markers
• reject obsolete markers instead of silently accepting them
• update to agent-eval 0.121.0 and agent-interface 0.28.0
• release the breaking persisted-format change as package 3.0.0

Verification

• pnpm install --frozen-lockfile
• pnpm lint
• pnpm typecheck
• pnpm test (282 passed, 10 skipped)
• pnpm build
• clean merge tree against origin/main

[tangletools]

✅ Auto-approved drewstone PR — 6e9a398c

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 · 2026-07-15T17:03:53Z}