fix(git): compute diff stats including uncommitted changes for dirty branches

[proboscis]

Summary

• Adds getWorkingTreeDiffStats function that computes diff from base branch to working tree (including uncommitted changes)
• Updates GetWorktreeStatus to use this function when the branch is dirty
• Adds unit tests to verify diff stats are properly computed for dirty branches

Problem

In orch-monitor TUI, the +/- columns showed - even when the branch column showed dirty. This happened because:

• getDiffStatsFast computed git diff --numstat origin/main...HEAD (only committed changes)
• When there are ONLY uncommitted changes, this shows 0/0, resulting in - in the TUI

Solution

When the branch is dirty, compute diff stats using git diff --numstat origin/main (without ...HEAD), which compares the base branch directly to the working tree and includes both committed AND uncommitted changes.

Evidence

Before fix (dirty branch with only uncommitted changes):

WorkingTreeDiff - Additions: 0, Deletions: 0

After fix:

WorkingTreeDiff - Additions: 18, Deletions: 1

Test output:

=== RUN   TestGetWorktreeStatus_DirtyWithDiffStats
--- PASS: TestGetWorktreeStatus_DirtyWithDiffStats (0.24s)
=== RUN   TestGetWorktreeStatus_DirtyWithBothCommittedAndUncommitted
--- PASS: TestGetWorktreeStatus_DirtyWithBothCommittedAndUncommitted (0.30s)
PASS

Files Changed

• internal/git/gogit.go: Added getWorkingTreeDiffStats, updated dirty branch handling
• internal/git/gogit_test.go: New tests for dirty worktree diff stats

Fixes: orch-401