feat(agent-interface): add AgentProfileDiff contract

[drewstone]

Summary

• Add a provider-neutral AgentProfileDiff artifact for full-profile improvement candidates.
• Add apply, prune, changed-axis, and schema validation helpers.
• Add a changeset for @tangle-network/agent-interface.

Checks

• pnpm --filter @tangle-network/agent-interface test
• pnpm --filter @tangle-network/agent-interface check-types
• pnpm --filter @tangle-network/agent-interface build
• git merge-tree --write-tree origin/main HEAD

[tangletools]

✅ Auto-approved drewstone PR — 79f8a09c

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 · 2026-07-04T01:01:12Z}