feat: built-in remote Action opens the git remote (ctrl+g) #65

Part of #61. Covers feature 4 (open the git remote with a keystroke).

What

A built-in remote Action, bound to ctrl+g, that opens the Project's remote in the browser. It is detached, so the Picker stays open.

• Remote choice: origin, else the first remote listed by git remote.
• URL rewrite to the https home page:
  • git@host:owner/repo.git → https://host/owner/repo
  • ssh://git@host[:port]/owner/repo.git → https://host/owner/repo
  • https://host/owner/repo.git → .git stripped
• Opens the repo home page, not the current branch.
• Opens with xdg-open on Linux, open on macOS.
• With no remote, or when the Project is not a git repo (an added directory, see feat(cli): cdd add and cdd forget for non-git Projects #68), it shows a one-line error in the Picker footer and the Picker stays open.

Acceptance

• URL rewriting is table-tested, including GitLab subgroups (owner/group/repo), ports and .git-less URLs.
• The no-remote and not-a-repo cases show the footer error.