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/down.go

@@ -74,8 +74,8 @@ func runDown(cmd *cobra.Command, args []string) error {
 	}
 
 	// Get session names
-	mayorSession, _ := getMayorSessionName()
-	deaconSession, _ := getDeaconSessionName()
+	mayorSession := getMayorSessionName()
+	deaconSession := getDeaconSessionName()
 
 	// 2. Stop Mayor
 	if err := stopSession(t, mayorSession); err != nil {