test: pin the edit symlink refusal, AppendRegular's clean check and Remove (#188)

Review round 1 on PR #189 (three tests stayed green with their guard
removed):
- TestEditRefusesANoteBehindASymlinkedDirectory had no .git, so Edit
  refused "not the sync source" before any path check. It now has one,
  plus a positive control.
- AppendRegular's unclean-path case was refused by a final symlink, not
  cleanRel. New test where ".." after a symlinked dir resolves to a real
  file, for AppendRegular and PutRegular.
- brainfs.Remove had no test; new one (refuses behind a symlinked dir,
  removes a final link but never its target, positive control).

Also: promote/reject create with O_EXCL (new brainfs.CreateRegular), so a
note that appears after the collision check is never overwritten;
gofmt fixes in provenance.go and wiki/log.go; dead wingDir and
putBrainFile removed.

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

ingestion/internal/brainfs/brainfs_test.go

@@ -453,3 +453,43 @@ func TestRealParentRefusesASymlinkedDirectoryOrUncleanPath(t *testing.T) {
 	assert.ErrorIs(t, b.RealParent("wiki/../raw/x.md"), ErrNotRegular)
 	assert.NoError(t, b.RealParent("wiki/x.md"), "positive control")
 }
+
+// cleanRel must refuse an unclean path on its own: here the ".." resolves,
+// after a symlinked directory, to a real regular file, so only the clean
+// check stands between the call and that file.
+func TestAppendRegularRefusesAnUncleanPathThatResolvesToARealFile(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "raw", "keep"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "raw", "v.md"), []byte("untouched"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("raw", "keep"), filepath.Join(dir, "indir")))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+	assert.ErrorIs(t, b.AppendRegular("indir/../v.md", []byte("x")), ErrNotRegular)
+	assert.ErrorIs(t, b.PutRegular("indir/../v.md", []byte("x")), ErrNotRegular)
+	got, _ := os.ReadFile(filepath.Join(dir, "raw", "v.md"))
+	assert.Equal(t, "untouched", string(got))
+}
+
+func TestRemoveRefusesAPathBehindASymlinkedDirectoryAndRemovesOnlyTheLink(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "raw"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "raw", "v.md"), []byte("keep"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki"), 0o755))
+	require.NoError(t, os.Symlink(filepath.Join("..", "raw"), filepath.Join(dir, "wiki", "alias")))
+	require.NoError(t, os.Symlink(filepath.Join("..", "raw", "v.md"), filepath.Join(dir, "wiki", "link.md")))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "wiki", "n.md"), []byte("n"), 0o644))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+
+	assert.ErrorIs(t, b.Remove("wiki/alias/v.md"), ErrNotRegular)
+	_, err = os.Stat(filepath.Join(dir, "raw", "v.md"))
+	assert.NoError(t, err, "the file behind the symlinked directory is kept")
+	require.NoError(t, b.Remove("wiki/link.md"))
+	_, err = os.Stat(filepath.Join(dir, "raw", "v.md"))
+	assert.NoError(t, err, "removing a final symlink never removes its target")
+	require.NoError(t, b.Remove("wiki/n.md"), "positive control")
+	_, err = os.Stat(filepath.Join(dir, "wiki", "n.md"))
+	assert.True(t, os.IsNotExist(err))
+}