Add circleci open command to next

[parkuman]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

=======

• Add circleci open command in internal/cmd/open/
• Register open command in internal/cmd/root/root.go
• Add github.com/pkg/browser dependency for opening URLs in the default browser
• Add tests for ProjectURL covering GitHub, Bitbucket, GitLab, and invalid slug cases

Rationale

=========

The circleci open command existed on the main branch (cmd/open.go) but was
not carried over during the migration to the next branch. I missed it! This restores the
feature, adapted to the new project structure and conventions (structured errors
via clierrors, git remote detection via internal/gitremote, gotest.tools
for tests).

Considerations

==============

• Reused the existing internal/gitremote.Detect() package rather than
  reimplementing git remote parsing. This is the same package used by
  circleci project follow.
• The old implementation only supported GitHub and Bitbucket. This version
  also supports GitLab since gitremote.Detect() already handles it.
• url.PathEscape is applied to slug segments as a safety net, though in
  practice VCS hosts restrict org/repo names to characters that don't need
  escaping.

Screenshots

============

After

Screen.Recording.2026-04-24.at.11.35.46.AM.mov

[parkuman]

Internal discussions ongoing here: https://circleci.slack.com/archives/C0AUWSGBFQD/p1777045015516229