fix: [ENG-3025] preserve structured cleanup-error signal + close docs gaps

Second round of PR review fixes (four findings on the previous fix
commit):

1. Switch cleanup-failure-post-success from warnings[] (free-text) to
   errors[] (structured kind). The previous fix moved
   response-file-delete-error into warnings, which buried the machine-
   readable kind under a substring-match. Now the cleanup error is
   appended to the dispatch envelope's errors[] so consumers can switch
   on `kind === 'response-file-delete-error'` programmatically. The
   `ok`/`status` fields stay as the daemon set them, so
   `{ok: true, status: 'done', errors: [{kind: 'response-file-delete-error', …}]}`
   is the success-with-cleanup-hiccup shape.

2. Update docs/curate-protocol.md row for response-file-delete-error to
   describe the new "non-terminal companion" lifecycle. Previously said
   "the curate is aborted" which mismatched the post-fix behavior.

3. Document the warnings[] field in the wire-envelope JSON skeleton.
   Was undocumented (pre-existing gap); this PR actively uses the field
   so now's the time to surface it.

4. Add a doc note under --delete-response-file explaining the
   correct-html re-authoring tax: on a validation-failed continuation,
   the file is unlinked but the session is still live, so the calling
   agent must author a fresh envelope and write a new file before the
   next continuation. Inline --response has no equivalent overhead.

Interactive verification: forced unlink failure via chmod-protected
parent dir. Result: status=done, ok=true, errors=[{kind:response-file-
delete-error, message:EACCES…}], warnings=None. Structured kind is
where consumers can find it.