fix: generated and promoted writes go through the brain root (#188, #158)

Writers that created or rewrote brain files by path followed symlinks:
- promote/reject: os.MkdirAll + os.WriteFile of the destination, then
  os.Remove of the raw source. Now PutRegular + brainfs.Remove; the source
  is kept when the write is refused.
- StampProvenanceFile (brain_edit's re-stamp): provenance.ApplyFile read
  and rewrote by path. Now ReadRegular + WriteRegular; ApplyFile, now
  unused, is removed.
- BuildWingIndex (_index.md) and wiki.RebuildIndex (wiki/index.md): now
  PutRegular.
- wiki.AppendLog (log.md) and AutoTunnel's fuzzy-candidate log
  (raw/tunnel-candidates-*.md): now AppendRegular; tunnel's fileExists,
  now unused, is removed.
- New brainfs.Remove: clean path, real parent, removes a final symlink
  itself, never its target.

brain_edit's own atomic rename was already refused behind a symlinked
directory by #187's read check; a guard test pins it.

Pipeline writes, BackfillRefs, the watcher's raw/ markers and the session
archive in cmd/server follow in a separate PR (#158).

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

ingestion/internal/api/promote.go

@@ -127,18 +127,16 @@ func PromoteNote(brainDir string, opts PromoteOptions) (string, error) {
 	}
 	fm.set("promoted_at", now)
 
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
-		return "", fmt.Errorf("create wing dir: %w", err)
-	}
-	// Write-then-delete: the source survives any write failure.
+	// Write-then-delete: the source survives any write failure. Both go
+	// through the brain root (hyperguild#188).
 	promoted, err := StampProvenance(fm.render()+body, opts.Origin, opts.WrittenBy)
 	if err != nil {
 		return "", err
 	}
-	if err := os.WriteFile(dest, []byte(promoted), 0o644); err != nil {
+	if _, err := putBrainFile(brainDir, dest, []byte(promoted)); err != nil {
 		return "", fmt.Errorf("write promoted note: %w", err)
 	}
-	if err := os.Remove(src); err != nil {
+	if err := removeBrainFile(brainDir, src); err != nil {
 		return "", fmt.Errorf("promoted note written but source removal failed: %w", err)
 	}
 
@@ -196,14 +194,12 @@ func RejectNote(brainDir string, opts RejectOptions) (string, error) {
 		fm.set("reject_reason", opts.Reason)
 	}
 
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
-		return "", fmt.Errorf("create rejected dir: %w", err)
-	}
-	// Write-then-delete: the source survives any write failure.
-	if err := os.WriteFile(dest, []byte(fm.render()+body), 0o644); err != nil {
+	// Write-then-delete: the source survives any write failure. Both go
+	// through the brain root (hyperguild#188).
+	if _, err := putBrainFile(brainDir, dest, []byte(fm.render()+body)); err != nil {
 		return "", fmt.Errorf("write rejected note: %w", err)
 	}
-	if err := os.Remove(src); err != nil {
+	if err := removeBrainFile(brainDir, src); err != nil {
 		return "", fmt.Errorf("rejected note written but source removal failed: %w", err)
 	}