feat: support try pr with gh url

[jellydn]

What

Added support for the try pr command and direct GitHub PR URL shorthand.

Why

Users need a way to easily check out GitHub pull requests into date-prefixed trial environments or local git worktrees without having to manually construct git clone/fetch commands or branch checkouts.

How

• Intercepted GitHub PR URLs in cmd_cd! to route them as PR URL shorthand.
• Implemented cmd_pr! command router to handle numeric PR IDs, shorthand repository/ID combinations (user/repo#id), and full GitHub PR URLs.
• Created shell script generators script_clone_pr (for remote repository checkouts) and script_worktree_pr (for local repository worktrees).
• Added test coverage in spec/tests/test_38_pr.sh and documented the command in spec/command_line.md.

Checklist

• Tests added or updated
• Documentation updated
• No breaking changes (or breaking changes documented above)

[jellydn]

How to Test This PR

You can test these changes locally using the following scenarios:

1. Setup dev symlink (optional but recommended)

To test the integration in your current shell:

ln -sf "$(pwd)/try.rb" ~/.local/bin/try-dev

2. Test explicit try pr command

You can test the command directly using a temporary trial path:

• With a numeric PR ID (run this from inside a local git repository to test worktree creation):

  TRY_PATH=$(mktemp -d) ./try.rb pr 121

• With a repo + ID shorthand:

  TRY_PATH=$(mktemp -d) ./try.rb pr tobi/try#121

• With a full GitHub PR URL:

  TRY_PATH=$(mktemp -d) ./try.rb pr https://github.com/tobi/try/pull/121

3. Test direct PR URL shorthand (no command keyword)

Passing the PR URL directly to try (or try cd) should automatically route it to the PR workflow:

TRY_PATH=$(mktemp -d) ./try.rb cd https://github.com/tobi/try/pull/121

4. Run automated tests

Run the spec test runner:

./spec/tests/runner.sh ./try.rb