Feature request: user-facing Git actions in the UI (Commit / Push), without involving the agent

## Summary
Mux has solid Git visibility (status/diff/review), but it still lacks a **user-facing way to perform Git actions in the UI**.

I’m requesting a small “Git actions” surface (button/panel) that lets **the human** do common actions like **Commit** and **Push** (and possibly Pull) directly from Mux—without asking the agent to do Git operations.

This is a real UX gap for “human in the loop” workflows: sometimes I want quick manual control for the final mile (commit/push), and the current options add friction.

## Motivation / Why this matters
- Sometimes I want the agent to *help write code*, but I want to **personally decide what gets committed and pushed**.
- The current workaround (“open terminal inside Mux / open lazygit / use a separate terminal in parallel”) works, but it’s noticeably slower and more context-switch-y than having a lightweight UI integration for the common operations.
- Other agentic orchestrator IDEs/tools (e.g., Conductor, Codex) have normalized the idea of “human commit/push buttons” as part of the workflow, so this request isn’t trying to invent a brand-new concept—Mux just has a real hole here.

## Related context
I’m aware of #1713, but that discussion was framed around **agent behavior / git support / what the agent should do**, not about giving the **user** manual Git controls in the UI:
https://github.com/coder/mux/issues/1713

This issue is explicitly: *don’t make the agent do Git for me; let me do it myself from the UI.*

## Proposed UX (rough)
A small Git panel (location flexible—sidebar, header, review tab, etc.) that shows:
- current branch + dirty state
- ahead/behind info (if available)
- quick actions:
- Stage / Unstage (optional but ideal)
- **Commit** (commit message input + commit button)
- **Push** (push current branch to configured remote)
- (optional) Pull / Fetch / Sync

## Non-goals / guardrails
- Not asking the agent to decide what to commit
- Not asking for auto-commit behavior
- Not asking for advanced git porcelain initially (rebase, interactive staging, force-push, etc.)
- A minimal first step would already be valuable: “Commit” + “Push” buttons with good error messaging.

## Workarounds I’m aware of (but still painful)
- Opening a terminal within Mux and running `git commit` / `git push`
- Running lazygit in Mux
- Using an external terminal in parallel

All of these work, but UI buttons would be faster + more convenient, especially for frequent small “commit/push” cycles after review.

## Acceptance criteria (suggested)
- I can commit from the UI (message entry, clear errors)
- I can push from the UI (clear success/failure feedback)
- The feature is explicitly **user-operated** (not “agent performs git operations automatically”)
- Works in the common runtimes Mux supports (local/ssh/docker), or clearly documents limitations per runtime

Thanks for considering—happy to help test or refine scope.