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

cmd/costs.go

@@ -161,4 +161,3 @@ func shortModel(name string) string {
 	}
 	return name
 }
-

cmd/root.go

@@ -76,7 +76,7 @@ func loadData() (*pipeline.LoadResult, error) {
 				fmt.Fprintf(os.Stderr, "  Cache unavailable, doing full parse\n")
 			}
 		} else {
-			defer cache.Close()
+			defer func() { _ = cache.Close() }()
 
 			cr, err := pipeline.LoadWithCache(flagDataDir, !flagNoSubagents, cache, progressFn)
 			if err != nil {

cmd/summary.go

@@ -51,7 +51,7 @@ func runSummary(_ *cobra.Command, _ []string) error {
 	fmt.Println()
 
 	// Build the summary table
-	rows := [][]string{
+	rows := [][]string{ //nolint:prealloc // appended conditionally below
 		{"Sessions", cli.FormatNumber(int64(stats.TotalSessions))},
 		{"Prompts", cli.FormatNumber(int64(stats.TotalPrompts))},
 		{"Total Time", cli.FormatDuration(stats.TotalDurationSecs)},
@@ -70,7 +70,7 @@ func runSummary(_ *cobra.Command, _ []string) error {
 	}
 
 	// Cost per day with delta
-	costDayStr := fmt.Sprintf("%s/day", cli.FormatCost(stats.CostPerDay))
+	costDayStr := cli.FormatCost(stats.CostPerDay) + "/day"
 	if prevStats.CostPerDay > 0 {
 		costDayStr += fmt.Sprintf("  (%s vs prev %dd)",
 			cli.FormatDelta(stats.CostPerDay, prevStats.CostPerDay), flagDays)