feat: status, sync and open across repositories #5

Goal

The state of every managed repository is visible in one command, and they can be refreshed
together.

Context

• Trigger: fifth milestone, independent of the health issue and runnable in parallel with it.
• Affected: internal/git, cmd/gitcd.

Reported and to be re-probed here: cloning an empty repository leaves an unborn HEAD, and
git status --porcelain=v2 --branch then prints # branch.oid (initial) and no # branch.ab
line. A branch without an upstream prints no # branch.ab line either. Both cases must render
as a state, never as a crash or a blank column.

sync uses a bounded pool of four. Serialising the first SSH connection per host was considered
and rejected: the agent and ControlMaster already handle the prompt storm.

Also in this milestone, small and parser driven: gitcd open opens the remote in a browser,
built from the existing remote parser.

Acceptance criteria

• gitcd status prints branch, dirty state and ahead or behind per repository.
• A repository with an unborn HEAD renders as a state, with a test.
• A branch without an upstream renders as a state, with a test.
• gitcd status --json prints the same as structured output.
• gitcd status exits non zero when any repository is dirty, behind the flag --strict.
• gitcd sync fetches every repository with a pool of four.
• gitcd sync reports per repository failures without aborting the run, and exits non zero
  when any failed.
• gitcd open [query] opens the remote in a browser and --print prints the URL instead.
• Tests cover the status rendering cases and the failure aggregation. No network.
• task check passes.

Out of scope

• prune for repositories whose upstream is gone.
• --filter=blob:none on clone.
• Pulling or rebasing. sync fetches only.