feat: add tier and session_log MCP tools

Adds two new MCP skill packages:
- internal/skills/org: exposes the tier tool, calling an injected TierFn
  for testability; returns current operating tier as structured JSON
- internal/skills/sessionlog: exposes the session_log tool, appending
  structured JSONL entries to brain/sessions/{session_id}.jsonl; requires
  session_id, wraps internal/session.Append

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/skills/sessionlog/handlers_test.go

@@ -0,0 +1,44 @@
+// internal/skills/sessionlog/handlers_test.go
+package sessionlog_test
+
+import (
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/supervisor/internal/skills/sessionlog"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHandle_SessionLog_AppendsEntry(t *testing.T) {
+	dir := t.TempDir()
+	s := sessionlog.New(sessionlog.Config{SessionsDir: dir})
+
+	args, _ := json.Marshal(map[string]any{
+		"session_id":   "sess-abc",
+		"skill":        "tdd_green",
+		"final_status": "pass",
+		"model_used":   "ollama/qwen3",
+		"duration_ms":  3000,
+	})
+	out, err := s.Handle(context.Background(), "session_log", args)
+	require.NoError(t, err)
+	var result map[string]string
+	require.NoError(t, json.Unmarshal(out, &result))
+	assert.Equal(t, "ok", result["status"])
+
+	// Verify file written
+	data, err := os.ReadFile(filepath.Join(dir, "sess-abc.jsonl"))
+	require.NoError(t, err)
+	assert.Contains(t, string(data), "tdd_green")
+}
+
+func TestHandle_SessionLog_RequiresSessionID(t *testing.T) {
+	s := sessionlog.New(sessionlog.Config{SessionsDir: t.TempDir()})
+	args, _ := json.Marshal(map[string]any{"skill": "tdd_red"})
+	_, err := s.Handle(context.Background(), "session_log", args)
+	assert.Error(t, err)
+}