feat(mirror): support SSH MIRRORS, multi-push origin and remote probe

- Switch MIRRORS to SSH-based URLs including custom ports/domains
  (GitHub, git.veen.world, code.cymais.cloud)
- Extend mirror IO:
  - load_config_mirrors filters empty values
  - read_mirrors_file now supports:
    * "name url" lines
    * "url" lines with auto-generated names from URL host (host[:port])
  - write_mirrors_file prints full preview content
- Enhance git_remote:
  - determine_primary_remote_url used for origin bootstrap
  - ensure_origin_remote keeps existing origin URL and
    adds all mirror URLs as additional push URLs
  - add is_remote_reachable() helper based on `git ls-remote --exit-code`
- Implement non-destructive remote mirror checks in setup_cmd:
  - `_probe_mirror()` wraps `git ls-remote` and returns (ok, message)
  - `pkgmgr mirror setup --remote` now probes each mirror URL and
    prints [OK]/[WARN] with details instead of placeholder text
- Add unit tests for mirror actions:
  - test_git_remote: default SSH URL building and primary URL selection
  - test_io: config + MIRRORS parsing including auto-named URL-only entries
  - test_setup_cmd: probe_mirror success/failure handling

https://chatgpt.com/share/693adee0-aa3c-800f-b72a-98473fdaf760

MIRRORS

@@ -1,3 +1,3 @@
-https://github.com/kevinveenbirkenbach/package-manager
-https://git.veen.world/kevinveenbirkenbach/package-manager
-https://code.infinito.nexus/kevinveenbirkenbach/package-manager
+git@github.com:kevinveenbirkenbach/package-manager.git
+ssh://git@git.veen.world:2201/kevinveenbirkenbach/pkgmgr.git
+ssh://git@code.cymais.cloud:2201/kevinveenbirkenbach/pkgmgr.git