fix: upgrade to go-git v6 to support worktreeConfig repos

[wowu]

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[Copilot]

Pull request overview

This PR upgrades the repository/ layer to use go-git v6 (alpha) and simplifies repository discovery/branch/origin resolution by relying on go-git’s built-in DetectDotGit and updated worktree/config handling, removing the previous manual worktree traversal logic.

Changes:

• Upgrade go-git dependency to github.com/go-git/go-git/v6 (alpha) and refresh module sums.
• Simplify Repository implementation: open via PlainOpenWithOptions{DetectDotGit:true}, use Head() for branch name, and Remote("origin") for origin URL.
• Update command-layer error handling to distinguish “no repository found” from other open errors, and update AGENTS documentation accordingly.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
repository/repository.go	Switches to go-git v6 and removes manual worktree/parent traversal logic in favor of DetectDotGit and go-git APIs.
repository/errors.go	Reworks sentinel errors (adds ErrNoRepository, removes unused error).
command/open.go	Updates error handling to use the new ErrNoRepository sentinel and improves messaging for other open failures.
command/list.go	Same as open.go: updated handling for ErrNoRepository and repository open failures.
go.mod	Replaces v5 dependencies with go-git/v6 and updates indirect requirements.
go.sum	Updates checksums to match the new module graph after the v6 upgrade.
AGENTS.md	Updates architecture docs to reflect the new approach and the rationale for pinning go-git v6 alpha.

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