fix(chat): delete pending server attachment when composer chip is removed

Post-merge forensic from PR #961 exact head 1115421a85e19420a1fdc6a03c3fc6797dd994de. chat-composer-attachments.tsx removes and clears attachment chips only in client state. Server pending rows remain and continue counting toward the thread limit; ten upload/remove cycles leave an empty composer that receives 409 on the next upload. Required fix: authenticated server-side pending deletion or cancellation semantics, idempotent cleanup, refresh-safe behavior, and end-to-end regression coverage. One issue, one PR. Related governance incident: #956. Coordination state: recorded but not dispatched because it is not in the current pnpm harness readiness queue.

Start with chat-composer-attachments.tsx and inspect PR #961 at head 1115421a85e19420a1fdc6a03c3fc6797dd994de to trace how chip removal currently updates client state. Reproduce ten upload/remove cycles, then locate the server-side pending attachment rows and existing end-to-end harness. Done means authenticated deletion or cancellation is idempotent, survives refresh, avoids the thread-limit 409, and has regression coverage.

Written by the indexing model from the issue text.