refactor(agent-interface): collapse candidate contracts

[drewstone]

Problem: the unlaunched candidate and improvement model exposed several historical shapes and 701 lines of compatibility code before any customer adoption.

Solution: keep one strict, unversioned contract for candidates, profile diffs, proposals, reviews, and evidence; delete the compatibility module and reject obsolete markers. Package semver remains the release boundary.

Local proof:

• pnpm build
• pnpm check-types
• pnpm test (546 tests passed)
• packed artifact excludes the deleted compatibility module
• clean merge with origin/main

[tangletools]

✅ Auto-approved drewstone PR — 5373c828

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 · 2026-07-15T16:13:02Z}