fix: linter compliance and code quality improvements across codebase

Address golangci-lint findings and improve error handling throughout:

Package doc comments:
- Add canonical "// Package X ..." comments to source, model, config,
  pipeline, cli, store, and main packages for godoc compliance.

Security & correctness:
- Fix directory permissions 0o755 -> 0o750 in store/cache.go Open()
  (gosec G301: restrict group write on cache directory)
- Fix config.Save() to check encoder error before closing file, preventing
  silent data loss on encode failure
- Add //nolint:gosec annotations with justifications on intentional
  patterns (constructed file paths, manual bounds checking, config fields)
- Add //nolint:nilerr on intentional error-swallowing in scanner WalkDir
- Add //nolint:revive on stuttering type names (ModelStats, ModelUsage)
  that would break too many call sites to rename

Performance (perfsprint):
- Replace fmt.Sprintf("%d", n) with strconv.FormatInt(n, 10) in format.go
  FormatTokens() and FormatNumber() hot paths
- Clean up redundant fmt.Sprintf patterns in FormatCost and FormatDelta

Code cleanup:
- Convert if-else chain to switch in parser.go skipJSONString() for clarity
- Remove unused indexedResult struct from pipeline/loader.go
- Add deferred cache.Close() in pipeline/bench_test.go to prevent leaks
- Add deferred cache.Close() in cmd/root.go data loading path
- Fix doc comment alignment in scanner.go decodeProjectName
- Remove trailing blank line in cmd/costs.go
- Fix duplicate "/day" suffix in cmd/summary.go cost-per-day formatting
- Rename shadowed variable 'max' -> 'maxVal' in cli/render.go Sparkline

internal/store/cache.go

@@ -1,3 +1,4 @@
+// Package store provides a SQLite-backed cache for parsed session data.
 package store
 
 import (
@@ -9,7 +10,7 @@ import (
 
 	"cburn/internal/model"
 
-	_ "modernc.org/sqlite"
+	_ "modernc.org/sqlite" // register sqlite driver
 )
 
 // Cache provides SQLite-backed session caching.
@@ -20,7 +21,7 @@ type Cache struct {
 // Open opens or creates the cache database at the given path.
 func Open(dbPath string) (*Cache, error) {
 	dir := filepath.Dir(dbPath)
-	if err := os.MkdirAll(dir, 0o755); err != nil {
+	if err := os.MkdirAll(dir, 0o750); err != nil {
 		return nil, fmt.Errorf("creating cache dir: %w", err)
 	}
 
@@ -30,7 +31,7 @@ func Open(dbPath string) (*Cache, error) {
 	}
 
 	if _, err := db.Exec(schemaSQL); err != nil {
-		db.Close()
+		_ = db.Close()
 		return nil, fmt.Errorf("creating schema: %w", err)
 	}
 
@@ -54,7 +55,7 @@ func (c *Cache) GetTrackedFiles() (map[string]FileInfo, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	result := make(map[string]FileInfo)
 	for rows.Next() {
@@ -74,7 +75,7 @@ func (c *Cache) SaveSession(s model.SessionStats, mtimeNs, sizeBytes int64) erro
 	if err != nil {
 		return err
 	}
-	defer tx.Rollback()
+	defer func() { _ = tx.Rollback() }()
 
 	now := time.Now().UTC().Format(time.RFC3339)
 	startTime := ""
@@ -147,7 +148,7 @@ func (c *Cache) LoadAllSessions() ([]model.SessionStats, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	var sessions []model.SessionStats
 	for rows.Next() {
@@ -195,7 +196,7 @@ func (c *Cache) LoadAllSessions() ([]model.SessionStats, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer modelRows.Close()
+	defer func() { _ = modelRows.Close() }()
 
 	// Build session index for fast lookup
 	sessionIdx := make(map[string]int)