feat(ingestion): extract WriteNote helper and add brain_write MCP tool

api.WriteNote captures the file-write logic that was previously inline
in Handler.Write. The existing HTTP endpoint now delegates to it; the
new MCP brain_write tool reuses the same function. Path-traversal
guard is strengthened to explicitly reject filenames containing path
separators or "..", so the rejection is surfaced before filepath.Base
strips the suspicious component (the previous defense-in-depth prefix
check became unreachable for these inputs after Base normalisation).
HTTP error code for caller-input errors shifts from 500 to 400, which
is semantically correct and not exercised by any existing test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/handlers_test.go

@@ -50,3 +50,47 @@ func TestBrainQueryReturnsResults(t *testing.T) {
 	text := content[0].(map[string]any)["text"].(string)
 	assert.Contains(t, text, "tdd.md")
 }
+
+func TestBrainWriteCreatesFile(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "# Test\n\nbody",
+		"filename": "test.md",
+		"type":     "note",
+		"domain":   "personal",
+	})
+	require.Nil(t, resp["error"])
+
+	got, err := os.ReadFile(filepath.Join(brainDir, "knowledge", "test.md"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "type: note")
+	assert.Contains(t, string(got), "domain: personal")
+	assert.Contains(t, string(got), "# Test")
+}
+
+func TestBrainWriteRejectsTraversal(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "x",
+		"filename": "../escape.md",
+	})
+	require.NotNil(t, resp["error"])
+}
+
+func TestBrainWriteAcceptsDoubleDotInName(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_write", map[string]any{
+		"content":  "x",
+		"filename": "notes..draft.md",
+	})
+	require.Nil(t, resp["error"])
+
+	_, err := os.Stat(filepath.Join(brainDir, "knowledge", "notes..draft.md"))
+	require.NoError(t, err, "filename with embedded .. should be allowed")
+}