fix(brain): provenance as a post-condition on every model-reachable write

The PR #147 review forged origin: static as a JWT principal through
three tools the first version never touched (brain_edit, brain_ingest_raw,
ingest_raw + brain_promote), and through YAML spellings a key-name match
missed (quoted, escaped, tagged, flow-mapped, merge keys, lone CR).

- api.StampProvenance removes every spelling of origin/written_by from
  the frontmatter and appends the real values. A line is dropped if
  EITHER yaml.v3 (decoding quotes, escapes, tags, flow maps) or the
  brain's line-oriented readers would read it as provenance; complex
  keys, orphan values and merge keys are dropped outright. Lone CRs in
  frontmatter become LF. Principal values that are not bare words are
  quoted.
- Applied as the last step of writeHallNote, UpdateNote, PromoteNote
  (re-attributed to the promoter), brain_edit (re-attributed to the
  editor) and both ingests (every page written or merged). REST
  /write and /promote carry no principal and stamp unknown.
- Ingest: newlines in scalar fields fold to spaces; a page type must be
  a plain lower-case name (../raw wrote outside wiki/, pre-existing).
- Audit: outcome is ok|error, never the error text, which echoed
  argument values.

Tests: every escape from the review is a table case asserting the
invariant on the file (parses as YAML, yaml.v3 and brain_get agree,
exactly the stamped lines). Plus REST /write, REST capture AuthPath,
body-line provenance in search, and quoted principals; two of the
review's surviving flips now go red. Three tests renamed so names claim
only what they assert.

Refs #145 #144

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/api/handler.go

@@ -194,11 +194,6 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	fmt.Fprintf(&fm, "hall: %s\n", opts.Hall)
 	fmt.Fprintf(&fm, "created_at: %s\n", time.Now().UTC().Format(time.RFC3339))
 	emitted["wing"], emitted["hall"], emitted["created_at"] = true, true, true
-	fmt.Fprintf(&fm, "origin: %s\n", provenanceOrigin(opts.Origin))
-	emitted["origin"], emitted["written_by"] = true, true
-	if opts.WrittenBy != "" {
-		fmt.Fprintf(&fm, "written_by: %s\n", opts.WrittenBy)
-	}
 
 	// writeField merges one key: opts.Content's own value (if the note already
 	// carries this field in its own frontmatter) always wins over the fallback,
@@ -238,7 +233,8 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	}
 	fm.WriteString("---\n")
 
-	if err := os.WriteFile(dest, []byte(fm.String()+body), 0o644); err != nil {
+	final := StampProvenance(fm.String()+body, opts.Origin, opts.WrittenBy)
+	if err := os.WriteFile(dest, []byte(final), 0o644); err != nil {
 		return "", fmt.Errorf("write: %w", err)
 	}
 	rel, _ := filepath.Rel(brainDir, dest)
@@ -613,7 +609,11 @@ func (h *Handler) Promote(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "invalid JSON")
 		return
 	}
-	rel, err := PromoteNote(h.brainDir, PromoteOptions(req))
+	// Field by field: this door carries no principal, so the promoted note is
+	// stamped origin: unknown and the body cannot set provenance (#145).
+	rel, err := PromoteNote(h.brainDir, PromoteOptions{
+		Filename: req.Filename, Wing: req.Wing, Hall: req.Hall, Slug: req.Slug,
+	})
 	if err != nil {
 		writeError(w, http.StatusBadRequest, err.Error())
 		return