fix(brainfs,api): PutRegular/AppendRegular; brain_write creates through the brain root (#188)

brain_write created a new note with os.MkdirAll + os.WriteFile by path,
so a symlinked wing or hall directory redirected the note elsewhere in
the brain or outside it.

New brainfs primitives for creating writes:
- PutRegular: create or replace. Missing parents are created one
  component at a time through the root and every existing component
  must be a real directory; an existing file is replaced only via
  WriteRegular's checks; a new one is created with O_EXCL and confirmed
  (Lstat + SameFile) before writing.
- AppendRegular: create-or-append with the same parent rule, writing
  only after confirming the path is the regular file it opened.
- RealParent: the clean-path + real-directory check, for writers that do
  their own atomic rename.

writeHallNote now uses PutRegular.

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

ingestion/internal/api/handler.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/claim"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
@@ -227,14 +228,22 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	// The directory is created only once the note is known to be writable:
 	// a refused write leaves nothing behind, not even an empty hall dir
 	// (brain#21; provenance refusal joins the envelope refusal here).
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
-		return "", fmt.Errorf("create hall dir: %w", err)
+	// Created through the brain root, never through a symlinked wing or
+	// hall directory (hyperguild#188).
+	rel, err := filepath.Rel(brainDir, dest)
+	if err != nil {
+		return "", fmt.Errorf("note path: %w", err)
 	}
-	if err := os.WriteFile(dest, []byte(final), 0o644); err != nil {
+	rel = filepath.ToSlash(rel)
+	bfs, err := brainfs.Open(brainDir)
+	if err != nil {
+		return "", fmt.Errorf("open brain: %w", err)
+	}
+	defer func() { _ = bfs.Close() }()
+	if err := bfs.PutRegular(rel, []byte(final)); err != nil {
 		return "", fmt.Errorf("write: %w", err)
 	}
-	rel, _ := filepath.Rel(brainDir, dest)
-	return filepath.ToSlash(rel), nil
+	return rel, nil
 }
 
 // scalarValues flattens frontmatter fields to key -> scalar value for