docs: [ENG-2697] correct dream.md sub-skill to match codebase

- Expand kind action notes (link/merge/prune/synthesize) with thresholds,
  candidate shapes, and accurate `related=` ref convention from curate.md.
- Add sample scan + finalize JSON envelopes with all candidate fields.
- Describe `path-exists` as a writer response on curate continuation
  (kickoff returns generate-html only; validation runs on the submitted HTML).
- Reframe undo: bit-exact restore of content + original mtime + sidecar
  signals; `brv dream undo` takes no arguments.
- Note that `dream sessions` / `dream cancel` JSON envelopes carry a
  `note` field disclosing v1 statelessness.