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/wiki/index.go

@@ -3,8 +3,6 @@ package wiki
 
 import (
 	"fmt"
-	"os"
-	"path/filepath"
 	"strings"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
@@ -50,8 +48,8 @@ func RebuildIndex(brainDir, date string) error {
 		sb.WriteString("\n")
 	}
 
-	dest := filepath.Join(brainDir, "wiki", "index.md")
-	return os.WriteFile(dest, []byte(sb.String()), 0o644)
+	// Written through the brain root, never through a symlink (#158).
+	return bfs.PutRegular("wiki/index.md", []byte(sb.String()))
 }
 
 // pageFirstSentence reads the page through the brain root, regular files