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.go

@@ -0,0 +1,83 @@
+// internal/session/session.go
+package session
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// Entry is one skill invocation record, appended to the session JSONL log.
+type Entry struct {
+	SessionID   string          `json:"session_id"`
+	Timestamp   time.Time       `json:"timestamp"`
+	Skill       string          `json:"skill"`
+	Phase       string          `json:"phase,omitempty"`
+	ProjectRoot string          `json:"project_root,omitempty"`
+	Input       json.RawMessage `json:"input,omitempty"`
+	Attempts    []Attempt       `json:"attempts,omitempty"`
+	FinalStatus string          `json:"final_status"`
+	FilePath    string          `json:"file_path,omitempty"`
+	ModelUsed   string          `json:"model_used,omitempty"`
+	DurationMs  int64           `json:"duration_ms,omitempty"`
+}
+
+// Attempt represents one subprocess invocation within a skill call.
+type Attempt struct {
+	Attempt       int    `json:"attempt"`
+	Model         string `json:"model"`
+	OutputSummary string `json:"output_summary,omitempty"`
+	RunnerOutput  string `json:"runner_output,omitempty"`
+	Verified      bool   `json:"verified"`
+}
+
+// Append writes entry as a single JSON line to sessionsDir/{sessionID}.jsonl.
+func Append(sessionsDir, sessionID string, entry Entry) error {
+	if err := os.MkdirAll(sessionsDir, 0o755); err != nil {
+		return fmt.Errorf("create sessions dir: %w", err)
+	}
+	path := filepath.Join(sessionsDir, sessionID+".jsonl")
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return fmt.Errorf("open session log: %w", err)
+	}
+	defer f.Close()
+
+	line, err := json.Marshal(entry)
+	if err != nil {
+		return fmt.Errorf("marshal entry: %w", err)
+	}
+	_, err = fmt.Fprintf(f, "%s\n", line)
+	return err
+}
+
+// Read returns all entries for sessionID. Returns empty slice if no log exists.
+func Read(sessionsDir, sessionID string) ([]Entry, error) {
+	path := filepath.Join(sessionsDir, sessionID+".jsonl")
+	f, err := os.Open(path)
+	if os.IsNotExist(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("open session log: %w", err)
+	}
+	defer f.Close()
+
+	var entries []Entry
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		if len(line) == 0 {
+			continue
+		}
+		var e Entry
+		if err := json.Unmarshal(line, &e); err != nil {
+			return nil, fmt.Errorf("parse entry: %w", err)
+		}
+		entries = append(entries, e)
+	}
+	return entries, scanner.Err()
+}