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/notes/edit_redirect_test.go

@@ -0,0 +1,25 @@
+package notes
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// hyperguild#188/#173: brain_edit's atomic rename must not be redirected by
+// a symlinked directory inside the brain.
+func TestEditRefusesANoteBehindASymlinkedDirectory(t *testing.T) {
+	b := t.TempDir()
+	victim := filepath.Join(b, "raw", "facts", "v.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(victim), 0o755))
+	require.NoError(t, os.WriteFile(victim, []byte("old text\n"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(b, "wiki", "w"), 0o755))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "raw", "facts"), filepath.Join(b, "wiki", "w", "alias")))
+	_, err := Edit(b, "wiki/w/alias/v.md", "old text", "new text", ScopeNotes)
+	assert.Error(t, err)
+	got, _ := os.ReadFile(victim)
+	assert.Equal(t, "old text\n", string(got))
+}