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.

go.sum

@@ -2,5 +2,5 @@ github.com/kevinburke/ssh_config v1.2.0 h1:x584FjTGwHzMwvHx18PXxbBVzfnxogHaAReU4
 github.com/kevinburke/ssh_config v1.2.0/go.mod h1:CT57kijsi8u/K/BOFA39wgDQJ9CxiF4nAY/ojJ6r6mM=
 github.com/moby/sys/mountinfo v0.7.2 h1:1shs6aH5s4o5H2zQLn796ADW1wMrIwHsyJ2v9KouLrg=
 github.com/moby/sys/mountinfo v0.7.2/go.mod h1:1YOa8w8Ih7uW0wALDUgT1dTTSBrZ+HiBLGws92L2RU4=
-golang.org/x/sys v0.1.0 h1:kunALQeHf1/185U1i0GOB/fy1IPRDDpuoOOqRReG57U=
-golang.org/x/sys v0.1.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.43.0 h1:Rlag2XtaFTxp19wS8MXlJwTvoh8ArU6ezoyFsMyCTNI=
+golang.org/x/sys v0.43.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=