fix(#4016): records that cite each other are created together, and a guessed id is refused

Sessions predicted the ids their next creates would get and wrote them into
plan bodies and reference notes before the records existed. The database
never collides; the sequence is shared by every session and user, so any
concurrent create took the guessed numbers and the references pointed at
someone else's records.

- create_records (new MCP tool) and start_planning(body=, steps=) create
  their records in ONE transaction: insert, flush for the real ids, rewrite
  {{ref:N}} / {{ref:milestone}} placeholders as #id "title", commit. No
  prediction, no waiting, no stub records left behind when a batch fails.
  Ids need not be consecutive and nothing depends on it.
- Every MCP create/update of a note, task or milestone refuses a #N sitting
  just above the highest assigned id (within 50): that can only be a guess.
  Refusal, not warning. Numbers far above the max (PRs, forge issues) pass.
- notes.build_note splits validation out of create_note so the batch
  validates records exactly as a single create does.
- writing-plans and using-scribe say to pass steps up front and never write
  an unassigned id; plugin version minted.

Integration test runs six concurrent batches and checks each resolves its
placeholders to its own records, and that a failing batch writes nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/scribe/mcp/tools/notes.py

@@ -22,6 +22,7 @@ from scribe.services import supersession as supersession_svc
 from scribe.services import systems as systems_svc
 from scribe.services import trash as trash_svc
 from scribe.services.note_usage import record_pulled
+from scribe.services.record_refs import refuse_guessed_ids
 
 
 async def list_notes(
@@ -169,6 +170,11 @@ async def create_note(
             instead (no duplicate bloat / no stale RAG copies). Set true only
             when you're sure this is a genuinely distinct note.
 
+    AN ID EXISTS ONLY ONCE A CREATE RETURNS IT. A body citing a `#N` that has
+    not been assigned yet is refused — every session and user draws from one
+    sequence, so an expected id is taken by whoever creates next. Records that
+    must cite each other: create_records, with {{ref:N}} placeholders.
+
     Returns the created note object including its assigned id, OR — when a
     near-duplicate is found and force is false — {"duplicate": true,
     "existing_id": ..., "message": ...} and nothing is created. A tagged
@@ -177,6 +183,7 @@ async def create_note(
     create the missing System, or deliberately leave it untagged.
     """
     uid = current_user_id()
+    await refuse_guessed_ids(title, body)
     if not force:
         dup = await dedup_svc.find_duplicate_note(
             uid, title, body, project_id=project_id or None,
@@ -269,6 +276,7 @@ async def update_note(
         fields["verify_with"] = verify_with
     if expires_when:
         fields["expires_when"] = expires_when
+    await refuse_guessed_ids(title, body)
     note = await notes_svc.update_note(
         uid, note_id, clear=clear or (), **fields
     )