feat(ingestion): implement brain_ingest MCP tool

Wraps pipeline.Run with the existing LLM client. Mirrors the HTTP
/ingest and /ingest-path semantics — accepts either path or
content+source, validates mutual exclusion, surfaces an explicit error
when the LLM client is not configured (test-mode).

ctx is threaded through to pipeline.Run for cancellation.

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

ingestion/internal/mcp/handlers_test.go

@@ -127,3 +127,46 @@ func TestBrainIngestRawDryRun(t *testing.T) {
 	_, err := os.Stat(filepath.Join(brainDir, "wiki", "concepts", "test-concept.md"))
 	assert.True(t, os.IsNotExist(err))
 }
+
+func TestBrainIngestRejectsBoth(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_ingest", map[string]any{
+		"content": "x",
+		"source":  "y",
+		"path":    "/tmp/foo.md",
+	})
+	require.NotNil(t, resp["error"])
+}
+
+func TestBrainIngestRequiresOne(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_ingest", map[string]any{})
+	require.NotNil(t, resp["error"])
+}
+
+func TestBrainIngestRejectsContentWithoutSource(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "brain_ingest", map[string]any{
+		"content": "x",
+	})
+	require.NotNil(t, resp["error"])
+}
+
+func TestBrainIngestRequiresLLMConfigured(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil) // nil pipelineCfg → no LLM
+
+	resp := toolCall(t, srv, "brain_ingest", map[string]any{
+		"content": "some content",
+		"source":  "test",
+	})
+	require.NotNil(t, resp["error"])
+	errObj := resp["error"].(map[string]any)
+	assert.Contains(t, errObj["message"].(string), "LLM not configured")
+}