Add git branch status context chip

[BobbyWang0120]

Description

Adds a new Git Branch Status context chip that shows the current branch plus upstream tracking counts in the prompt/status chip UI:

• Shows branch ↑ahead ↓behind when the branch has an upstream and counts are available.
• Falls back to branch-only display when no upstream is configured or counts cannot be computed.
• Reuses GitRepoStatusModel metadata so local repo/file watcher updates refresh the chip without silently fetching from the network.
• Makes the chip available in both the general prompt chip configurator and the third-party CLI agent toolbar configurator.

Linked Issue

Fixes #5766

• The linked issue is labeled ready-to-spec or ready-to-implement.
  • Current state: Warp prompt needs an indicator for Git status ahead/behind remote #5766 is already triaged and closely matches this PR, but it does not yet have a readiness label.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).
  • Manual UI testing has been completed locally; screenshot/video can be attached before marking this PR ready.

Testing

• I have manually tested my changes locally with ./script/run

Automated checks run locally:

• cargo fmt -- --check
• cargo clippy -p warp --all-targets --tests -- -D warnings
• cargo check -p warp
• cargo nextest run -p warp -E 'test(parse_ahead_behind_counts_accepts_git_rev_list_output) | test(test_git_branch_tracking_status_displays_ahead_and_behind_when_upstream_exists) | test(test_git_branch_tracking_status_displays_branch_only_without_upstream) | test(test_git_branch_tracking_status_displays_branch_only_when_counts_are_unavailable) | test(test_git_branch_status_chip_display_value_uses_git_prefix) | test(test_git_status_change_updates_branch_status_chip_value)'

Manual verification:

• Enabled the new chip in the normal prompt chip configuration UI.
• Enabled the new chip in the third-party CLI agent toolbar configuration UI.
• Verified the chip renders the branch name with ahead/behind commit counts.

Screenshots / Videos

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-IMPROVEMENT: Added a Git Branch Status prompt chip that shows branch name with ahead/behind commit counts.

Co-Authored-By: Warp agent@warp.dev

[oz-for-oss]

@BobbyWang0120

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: Warp prompt needs an indicator for Git status ahead/behind remote #5766

• Required readiness label: ready-to-implement

Readiness check:

• Warp prompt needs an indicator for Git status ahead/behind remote #5766: missing ready-to-implement; readiness labels present: none

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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[oz-for-oss]

@BobbyWang0120

This PR is not linked to an issue that is marked with ready-to-implement.

Issue-state enforcement details:

• Associated same-repo issues checked: #5766

• Required readiness label: ready-to-implement

Readiness check:

• #5766: missing ready-to-implement; readiness labels present: none

To continue, link this PR to a same-repo issue such as Closes #123 in the PR description, and make sure that issue has ready-to-implement.

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[BobbyWang0120]

@oss-maintainers This PR implements #5766 and includes screenshots plus local/manual testing. Could you please review whether #5766 can be marked ready-to-implement and approve the pending workflows when appropriate?