feat(ingestion): implement session_log MCP tool

Appends a JSON line to brainDir/sessions/<session_id>.jsonl using the
session package copied in Task 2. Required for upcoming pass-rate
logging.

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

ingestion/internal/mcp/handlers_test.go

@@ -170,3 +170,27 @@ func TestBrainIngestRequiresLLMConfigured(t *testing.T) {
 	errObj := resp["error"].(map[string]any)
 	assert.Contains(t, errObj["message"].(string), "LLM not configured")
 }
+
+func TestSessionLogAppends(t *testing.T) {
+	brainDir := t.TempDir()
+	srv := mcp.NewServer(brainDir, nil, nil)
+
+	resp := toolCall(t, srv, "session_log", map[string]any{
+		"session_id":   "session-x",
+		"skill":        "tdd",
+		"phase":        "red",
+		"final_status": "ok",
+	})
+	require.Nil(t, resp["error"])
+
+	got, err := os.ReadFile(filepath.Join(brainDir, "sessions", "session-x.jsonl"))
+	require.NoError(t, err)
+	assert.Contains(t, string(got), `"skill":"tdd"`)
+	assert.Contains(t, string(got), `"phase":"red"`)
+}
+
+func TestSessionLogRequiresSessionID(t *testing.T) {
+	srv := mcp.NewServer(t.TempDir(), nil, nil)
+	resp := toolCall(t, srv, "session_log", map[string]any{"skill": "tdd"})
+	require.NotNil(t, resp["error"])
+}