fix: Address golangci-lint errors (errcheck, gosec) (#76)

Apply PR #76 from dannomayernotabot:

- Add golangci exclusions for internal package false positives
- Tighten file permissions (0644 -> 0600) for sensitive files
- Add ReadHeaderTimeout to HTTP server (slowloris prevention)
- Explicit error ignoring with _ = for intentional cases
- Add //nolint comments with justifications
- Spelling: cancelled -> canceled (US locale)

Co-Authored-By: dannomayernotabot <noreply@github.com>

🤖 Generated with Claude Code

internal/cmd/audit.go

@@ -159,7 +159,7 @@ func parseDuration(s string) (time.Duration, error) {
 }
 
 // collectGitCommits queries git log for commits by the actor.
-func collectGitCommits(townRoot, actor string, since time.Time) ([]AuditEntry, error) {
+func collectGitCommits(townRoot, actor string, since time.Time) ([]AuditEntry, error) { //nolint:unparam // error return kept for future use
 	var entries []AuditEntry
 
 	// Build git log command