🤖 fix: ignore stale origin/HEAD in git status scripts

[ammar-agent]

Context

#1414 (merged) unified the review-base selector with the git status indicator so the diff view and divergence numbers use the same base.

However, mux workspaces can be created from git bundles, and those clones can inherit a stale refs/remotes/origin/HEAD (e.g. pointing at a feature branch). If we use origin/HEAD as the base (explicitly or implicitly), ahead/behind can be computed against the wrong remote ref.

What this PR changes (follow-up)

• generateGitStatusScript()
  • Treats origin/HEAD as auto-detect even if configured as the base ref
  • Uses configured origin/<branch> when present (with main↔master fallback if the ref doesn't exist locally)
  • When auto-detecting, ignores suspicious origin/HEAD values and prefers origin/main or origin/master
• GIT_FETCH_SCRIPT
  • Derives the remote default branch + SHA via git ls-remote --symref origin HEAD (instead of local origin/HEAD)
• Storybook mock
  • Detects the consolidated git status script via output markers (less brittle)
• Tests
  • Adds unit coverage for parseGitShowBranchForStatus

Regression risk

• Auto-detection uses more heuristics; repos without origin/main or origin/master may fall back differently than before.
• GIT_FETCH_SCRIPT now depends on git ls-remote --symref origin HEAD; if the remote is unreachable/slow, fetch will be skipped and statuses may remain stale until a later refresh/manual fetch.
• If a user intentionally configured origin/HEAD as a base ref, it will now be treated as auto-detect (by design) to avoid stale/incorrect divergence.

Testing

• make static-check

---

Generated with mux • Model: openai:gpt-5.2 • Thinking: xhigh

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[ammar-agent]

@codex review

Addressed feedback:

1. Simplified the review base logic - git status now just uses the global default (review-default-base, which defaults to origin/main)
2. Removed the unnecessary project-scoping attempt since the review panel stores per-workspace and the global default is sufficient for the git status indicator

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".