refactor: harden ssh_config handling, mount path, and CLI UX from audit findings

Three rounds of audit-driven hardening, fully documented in DECISIONS.md:

- argv hardening: validate HostName/User/IdentityFile via allowlist regexes,
  parse Port via strconv.Atoi, surface ssh_config parse errors instead of
  silently swallowing them. Switch -o kernel_cache to auto_cache for network-
  FS correctness, pin StrictHostKeyChecking=accept-new.
- LOW-severity cleanup: -v verbose flag (default output is just the mount
  path), run_editor returns errors and main exits 7 on failure, ABOUTME
  headers, golang.org/x/sys v0.43.0 (go 1.25.0).
- Defense-in-depth + UX: rxIdentityFile first-character anchor rejects
  leading "-"/"."/":"/etc., verify_mount_dir resolves base via EvalSymlinks
  and refuses pre-existing symlinks at the mount path, flag.Usage shows the
  positional <Host> argument, run_editor uses cmd.Start() so cold-start
  Sublime does not block the terminal.
- CI: empty-PKGVER guard in update-pkgver workflow.
- Tests: verify_mount_dir path-traversal + symlink-reject coverage,
  rxHostUser/rxIdentityFile boundary cases.

.gitea/workflows/update-pkgver.yaml

@@ -36,6 +36,10 @@ jobs:
           fi
 
           PKGVER=$(cat /tmp/pkgver)
+          if [ -z "$PKGVER" ]; then
+            echo "ERROR: PKGVER from previous step is empty."
+            exit 1
+          fi
           git clone http://gitea:3000/nevaforget/moonarch-pkgbuilds.git pkgbuilds
           cd pkgbuilds