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/frontmatter.go

@@ -95,18 +95,6 @@ func (f *frontmatter) set(key, value string) {
 	f.lines = append(f.lines, fmLine{key: key, value: value})
 }
 
-// del removes every top-level line for key. Continuation lines of a
-// multi-line value are not tracked per key, so del is only for scalars.
-func (f *frontmatter) del(key string) {
-	kept := f.lines[:0]
-	for _, l := range f.lines {
-		if l.key != key {
-			kept = append(kept, l)
-		}
-	}
-	f.lines = kept
-}
-
 // render serialises the frontmatter back into a `---`-fenced block. An
 // empty frontmatter renders to the empty string so bodies without a
 // header stay header-less.