fix(ingestion): confine /ingest-path and brain_ingest paths to brain/raw/

Both read a caller-chosen server path with extract.Text, so any file the
pod could read (the ServiceAccount token, /etc/passwd) could be ingested
into the brain and read back with brain_query.

ResolveIngestPath resolves relative paths against brain/raw/ (the inbox
the watcher already owns), evaluates symlinks, and refuses anything
that lands outside. A directory ingest re-resolves every walked file,
so a symlinked file inside raw/ cannot lead out. Existing ingest-path
tests moved their fixtures under brain/raw/.

Refs #149

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

ingestion/internal/api/ingestpath_root.go

@@ -0,0 +1,37 @@
+package api
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// ErrOutsideIngestRoot means an ingest path does not resolve inside brain/raw/.
+var ErrOutsideIngestRoot = errors.New("ingest path must be inside brain/raw/")
+
+// ResolveIngestPath confines a caller-supplied ingest path to brain/raw/, the
+// inbox the watcher already owns (hyperguild#149). Relative paths resolve
+// against that root; symlinks are evaluated before the containment check, so
+// a link inside raw/ that points elsewhere is refused. The path must exist.
+func ResolveIngestPath(brainDir, p string) (string, error) {
+	if strings.TrimSpace(p) == "" {
+		return "", fmt.Errorf("%w: path is empty", ErrOutsideIngestRoot)
+	}
+	root, err := filepath.EvalSymlinks(filepath.Join(brainDir, "raw"))
+	if err != nil {
+		return "", fmt.Errorf("%w: inbox unavailable: %v", ErrOutsideIngestRoot, err)
+	}
+	if !filepath.IsAbs(p) {
+		p = filepath.Join(root, p)
+	}
+	real, err := filepath.EvalSymlinks(filepath.Clean(p))
+	if err != nil {
+		return "", fmt.Errorf("%w: %v", ErrOutsideIngestRoot, err)
+	}
+	rel, err := filepath.Rel(root, real)
+	if err != nil || rel == ".." || strings.HasPrefix(rel, ".."+string(filepath.Separator)) || filepath.IsAbs(rel) {
+		return "", ErrOutsideIngestRoot
+	}
+	return real, nil
+}