feat(ingestion): add session package for JSONL log persistence

Copy of internal/session from the supervisor module — the ingestion
service needs it for the upcoming session_log MCP tool. The supervisor
copy will be removed in the supervisor-retirement plan; until then
the two packages are intentionally identical and pinned (no edits).

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

ingestion/internal/session/session_test.go

@@ -0,0 +1,50 @@
+package session_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/session"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAppendAndRead(t *testing.T) {
+	dir := t.TempDir()
+	sid := "test-session"
+
+	e1 := session.Entry{
+		SessionID:   sid,
+		Timestamp:   time.Now().UTC().Truncate(time.Second),
+		Skill:       "tdd",
+		Phase:       "red",
+		FinalStatus: "ok",
+	}
+	e2 := session.Entry{
+		SessionID:   sid,
+		Timestamp:   time.Now().UTC().Truncate(time.Second),
+		Skill:       "tdd",
+		Phase:       "green",
+		FinalStatus: "ok",
+	}
+
+	require.NoError(t, session.Append(dir, sid, e1))
+	require.NoError(t, session.Append(dir, sid, e2))
+
+	got, err := session.Read(dir, sid)
+	require.NoError(t, err)
+	require.Len(t, got, 2)
+	assert.Equal(t, "red", got[0].Phase)
+	assert.Equal(t, "green", got[1].Phase)
+
+	_, statErr := os.Stat(filepath.Join(dir, sid+".jsonl"))
+	require.NoError(t, statErr, "session file should exist on disk")
+}
+
+func TestReadMissingReturnsEmpty(t *testing.T) {
+	got, err := session.Read(t.TempDir(), "nope")
+	require.NoError(t, err)
+	assert.Empty(t, got)
+}