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/pipeline/aggregator.go

@@ -1,3 +1,4 @@
+// Package pipeline orchestrates session loading, caching, and metric aggregation.
 package pipeline
 
 import (

internal/pipeline/bench_test.go

@@ -79,7 +79,7 @@ func BenchmarkLoadWithCache(b *testing.B) {
 	if err != nil {
 		b.Fatal(err)
 	}
-	defer cache.Close()
+	defer func() { _ = cache.Close() }()
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {

internal/pipeline/loader.go

@@ -67,11 +67,6 @@ func Load(claudeDir string, includeSubagents bool, progressFn ProgressFunc) (*Lo
 		numWorkers = len(toProcess)
 	}
 
-	type indexedResult struct {
-		idx    int
-		result source.ParseResult
-	}
-
 	work := make(chan int, len(toProcess))
 	results := make([]source.ParseResult, len(toProcess))
 	var wg sync.WaitGroup