feat: add session log package (append/read JSONL)

Introduces internal/session with Entry and Attempt types, Append
(O_APPEND JSONL writer) and Read (line scanner, nil on missing file).
Raw material for retrospective and trainer workers.

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

internal/session/session_test.go

@@ -0,0 +1,63 @@
+// internal/session/session_test.go
+package session_test
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/mathiasbq/supervisor/internal/session"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAppend_WritesJSONLEntry(t *testing.T) {
+	dir := t.TempDir()
+	entry := session.Entry{
+		SessionID:   "test-session-1",
+		Timestamp:   time.Now().UTC(),
+		Skill:       "tdd_green",
+		Phase:       "green",
+		ProjectRoot: "/tmp/myproject",
+		FinalStatus: "pass",
+		ModelUsed:   "ollama/qwen3",
+		DurationMs:  5000,
+	}
+
+	require.NoError(t, session.Append(dir, "test-session-1", entry))
+
+	path := filepath.Join(dir, "test-session-1.jsonl")
+	data, err := os.ReadFile(path)
+	require.NoError(t, err)
+
+	var got session.Entry
+	require.NoError(t, json.Unmarshal(data, &got))
+	assert.Equal(t, "test-session-1", got.SessionID)
+	assert.Equal(t, "tdd_green", got.Skill)
+	assert.Equal(t, "pass", got.FinalStatus)
+}
+
+func TestAppend_AppendsMultipleEntries(t *testing.T) {
+	dir := t.TempDir()
+	for i := 0; i < 3; i++ {
+		require.NoError(t, session.Append(dir, "s1", session.Entry{
+			SessionID:   "s1",
+			Timestamp:   time.Now().UTC(),
+			Skill:       "tdd_red",
+			FinalStatus: "pass",
+		}))
+	}
+
+	entries, err := session.Read(dir, "s1")
+	require.NoError(t, err)
+	assert.Len(t, entries, 3)
+}
+
+func TestRead_EmptyWhenNoFile(t *testing.T) {
+	dir := t.TempDir()
+	entries, err := session.Read(dir, "missing")
+	require.NoError(t, err)
+	assert.Empty(t, entries)
+}