docs: di-bench improvement loop

[aural-psynapse]

Changes

Docs

• AGENTS.md — note it is the canonical agent entry point (mirrored as the provably.ai machine view), and make explicit that claims must come from the agent's own SourceryKitAgentResponse output, not be hand-assembled.
• docs/handoff.md — add a SourceryKitAgentResponse section describing the typed output and its ClaimedValue fields (path, value, sourcerykit_ref), ordered type → component. Correct the claimed_value payload row from Any to list[ClaimedValue].
• cookbooks/openai_agents — accept OPENROUTER_API_KEY as well as MODEL_API_KEY, so a verbatim copy authenticates against an OpenAI-compatible gateway (OpenRouter).

Code

• evaluate_handoff / build_handoff_payload — when a claim can't be resolved it was dropped with only a log line, so a payload that ended up with zero claims surfaced as ERROR with an empty errors list. The drop reason now travels on the payload (build_errors) and evaluate_handoff returns it in errors, so a zero-claim result explains itself.

Why

These changes come out of a discoverability loop that runs a fresh agent from provably.ai to a working SourceryKit integration and turns each observed failure into one fix. Runs kept failing at the same points: agents couldn't find the entry doc, hand-assembled claims instead of binding SourceryKitAgentResponse, couldn't authenticate the cookbook with the key they had, and hit an ERROR with no reason when claims failed to resolve. Each item above addresses one of those. (The separate preprocess bug that blocked verification end-to-end is fixed in #71.)

[SimoneBottoni]

LGTM

[SimoneBottoni]

LGTM