Support commits from a linked worktree

[artem-y]

In this PR:

• updated the tool to support making commits from linked worktree
• added a test for a scenario when commit is being made from a linked worktree
• cleaned up extra whitespaces

[Copilot]

Pull request overview

This PR updates the commit CLI to correctly support commits made from linked Git worktrees by using go-git’s common .git directory support, and adds an end-to-end test to cover committing from a linked worktree. It also includes minor whitespace cleanups and removes an unused constant.

Changes:

• Update repository opening and HEAD/branch detection to work in linked worktrees (EnableDotGitCommonDir, use plumbing.HEAD reference).
• Add an e2e test that creates a linked worktree and commits from it.
• Clean up whitespace and remove an unused helper constant.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.

File	Description
cmd/commit/main.go	Opens repos with support for common git dirs and derives branch name via go-git refs (worktree-compatible).
e2e.sh	Adds a linked-worktree commit test and a helper for removing worktrees; whitespace normalization.
internal/helpers/constants.go	Removes now-unused HEAD_REF_PREFIX constant.
docs/README.md	Whitespace cleanup in JSON example (but reveals a JSON validity issue).

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