feat: add support for git worktrees

[patrickschroeter]

Add support for git worktrees (and submodules)

Summary

b5 failed to detect the project root when run from inside a git worktree or submodule. In a normal checkout .git is a directory, but in linked worktrees and submodules it is a file containing a gitdir: pointer to the real git directory. The old detect_git check required .git to be a directory, so path detection silently failed in those setups.

Changes

• b5/lib/detect.py — detect_git() now accepts .git whether it's a directory (normal checkout) or a file (worktree/submodule), by dropping the os.path.isdir() requirement.
• b5/tests/lib/test_detect.py — new test module covering normal checkouts, worktrees, the no-git case, and end-to-end project-path detection from a subdirectory of a worktree.
• CHANGELOG.md — new 1.4.2 entry describing the fix.
• RELEASE.md — new document describing the release process (version bump, tagging, GitHub Release, automated PyPI publish via GitHub Actions).