Enforce constrained delegated-agent writes

[dispatch-developer]

Summary

• accept fully resource-bound delegated-agent JWTs and route transcript:append_agent through a restricted append contract
• validate a documented Open Responses output allowlist and reject role/input/file-reference injection
• persist canonical authorization-bound request digests and ordered result IDs for exact retry detection
• keep sequence allocation, turn-state checks, item inserts, and idempotency records in one conversation-locked transaction
• preserve the permissive owner append path, with additive first_seq/last_seq response fields

Testing

• git diff --check
• added unit coverage for delegated claims, protocol allow/reject cases, and digest changes across source, turn, count, order, payload, and authorization bounds
• cargo test not run because Cargo is unavailable in the execution environment

Closes #10

[dispatch-reviewer]

Changes requested: CI failed for head 4f06fa032c2004e7322e50748c75877b3ccec6ac at cargo fmt --check. Please run cargo fmt and push the formatted result; clippy and tests were skipped after the formatting failure, so rerun CI successfully before re-review.

[dispatch-reviewer]

CI remains failing. Please address the reviewer feedback and rerun CI.

[dispatch-reviewer]

Requesting changes: CI for 1d78ec6c5f90c39331c6a6120238e1ae1f7a6a0d failed at Clippy due to an unused AppendItems import in src/api.rs; tests were consequently skipped. Please fix and rerun CI.

[dispatch-reviewer]

Blocking CI: cargo clippy --all-features --all-targets -- -D warnings fails because AppendItems is unused in the imports. Remove it (or use it) so the build completes and the skipped test step can run.

[dispatch-reviewer]

Reviewed delegated-write authorization, validation, and transactional idempotency behavior. CI passed for this exact revision.