feat: copy open PR URL

[ronen-hoffer]

Summary

• add a :copy-url command for copying the currently open PR or MR URL
• show a clear warning when the command is used outside PR mode
• document the command in the README, keybindings guide, and in-app help

Motivation

When I go over my team PRs my current flow is

1. Reviewing myself
2. Posting in draft
3. Run a agent to polish my lazy comments and suggest more

Copying the URL just makes me stick to the terminal and simply switching from tuicr to my agent terminal session.

Testing

• cargo fmt -- --check
• cargo test handler::command_tests
• cargo clippy --all-targets -- -D warnings
• full cargo test: 1200 passed; one environment-dependent existing test failed because the installed Git rejects the relativeworktrees repository extension (vcs::git::libgit2::tests::should_discover_worktree_with_relativeworktrees_extension)

[agavra]

Looks like we need to update this now that we have bitbucket support