fix: session log error handling and scanner buffer size

- Replace deprecated os.IsNotExist with errors.Is(err, fs.ErrNotExist)
- Capture Close error in Append by calling it explicitly instead of defer
- Increase scanner buffer to 1 MB per line to handle large JSONL entries

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

internal/session/session.go

@@ -4,7 +4,9 @@ package session
 import (
 	"bufio"
 	"encoding/json"
+	"errors"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"time"
@@ -44,21 +46,27 @@ func Append(sessionsDir, sessionID string, entry Entry) error {
 	if err != nil {
 		return fmt.Errorf("open session log: %w", err)
 	}
-	defer f.Close()
 
 	line, err := json.Marshal(entry)
 	if err != nil {
+		_ = f.Close()
 		return fmt.Errorf("marshal entry: %w", err)
 	}
-	_, err = fmt.Fprintf(f, "%s\n", line)
-	return err
+	if _, err = fmt.Fprintf(f, "%s\n", line); err != nil {
+		_ = f.Close()
+		return fmt.Errorf("write entry: %w", err)
+	}
+	if err = f.Close(); err != nil {
+		return fmt.Errorf("close session log: %w", err)
+	}
+	return nil
 }
 
 // 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) {
+	if errors.Is(err, fs.ErrNotExist) {
 		return nil, nil
 	}
 	if err != nil {
@@ -68,6 +76,7 @@ func Read(sessionsDir, sessionID string) ([]Entry, error) {
 
 	var entries []Entry
 	scanner := bufio.NewScanner(f)
+	scanner.Buffer(make([]byte, 0, 256*1024), 1<<20) // up to 1 MB per line
 	for scanner.Scan() {
 		line := scanner.Bytes()
 		if len(line) == 0 {