Add git worktree support

[arbourd]

Enhance Git repository detection by using 'git rev-parse --show-toplevel'
and '--absolute-git-dir', enabling native support for Git worktrees and
bare repositories. Current reference detection now correctly falls back
to full commit SHAs when in a detached HEAD state.

Modernize internal logic using Go 1.24+ iterators and simplified data
structures for provider loading.

[Copilot]

Pull request overview

Enhances repository detection and URL generation to support Git worktrees and bare repositories by switching to git rev-parse --show-toplevel / --absolute-git-dir, and modernizes provider loading and related tests.

Changes:

• Update git path detection to use rev-parse --show-toplevel with a bare-repo fallback to --absolute-git-dir.
• Improve detached-HEAD handling by falling back to full commit SHA for the current ref.
• Refactor provider loading to simplify data structures and make tests order-independent; add a worktree-focused integration test.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
open/provider.go	Simplifies provider aggregation while parsing git config provider entries.
open/provider_test.go	Makes provider comparisons deterministic by sorting slices before comparison.
open/open.go	Switches repo root detection to rev-parse to support worktrees/bare repos and tightens provider matching by host.
open/open_test.go	Adds a worktree test and extends remote parsing tests for local-path remotes.
gitw/gitw.go	Introduces Toplevel / AbsoluteGitDir, refactors git invocation options, and improves detached-HEAD ref resolution.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.