Implement Git status, diff, stage, unstage, and discard flows #55

Problem

The workbench needs the minimum safe Git loop for inspecting and managing local changes without leaving the app.

Scope

• Track status, diff, stage, unstage, and discard behavior for text-based work
• Cover how these flows appear in the workbench and inspector panes
• Keep state refresh explicit and understandable

Out of scope

• Remote operations
• Commit history and worktree management beyond the minimum local-change loop

Implementation notes

• Make Git state a first-class workbench surface, not a hidden tool-call side effect
• Align Git review behavior with file diff review and session auditability
• Keep destructive actions explicit

Definition of Done

• The issue defines the minimum safe local Git operations clearly
• Later implementation can add the Git surface without inventing its operator behavior

Verification

• Review the issue against the feature spec and workbench issues

Dependencies

• Parent epic: [Epic] Add the built-in Git workspace experience #18