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/cli/format.go

@@ -1,8 +1,10 @@
+// Package cli provides formatting and rendering utilities for terminal output.
 package cli
 
 import (
 	"fmt"
 	"math"
+	"strconv"
 	"strings"
 )
 
@@ -22,14 +24,14 @@ func FormatTokens(n int64) string {
 	case abs >= 1_000:
 		return fmt.Sprintf("%.1fK", float64(n)/1_000)
 	default:
-		return fmt.Sprintf("%d", n)
+		return strconv.FormatInt(n, 10)
 	}
 }
 
 // FormatCost formats a USD cost value.
 func FormatCost(cost float64) string {
 	if cost >= 1000 {
-		return fmt.Sprintf("$%s", FormatNumber(int64(math.Round(cost))))
+		return "$" + FormatNumber(int64(math.Round(cost)))
 	}
 	if cost >= 100 {
 		return fmt.Sprintf("$%.0f", cost)
@@ -66,7 +68,7 @@ func FormatNumber(n int64) string {
 		return "-" + FormatNumber(-n)
 	}
 
-	s := fmt.Sprintf("%d", n)
+	s := strconv.FormatInt(n, 10)
 	if len(s) <= 3 {
 		return s
 	}
@@ -95,9 +97,9 @@ func FormatPercent(f float64) string {
 func FormatDelta(current, previous float64) string {
 	delta := current - previous
 	if delta >= 0 {
-		return fmt.Sprintf("+%s", FormatCost(delta))
+		return "+" + FormatCost(delta)
 	}
-	return fmt.Sprintf("-%s", FormatCost(-delta))
+	return "-" + FormatCost(-delta)
 }
 
 // FormatDayOfWeek returns a 3-letter day abbreviation from a weekday number.