Add rtk git checkout support

Problem

rtk discover shows 617 git checkout invocations in the last 30 days, all unhandled. Common forms:

• git checkout -b <branch>
• git checkout <branch>
• git checkout HEAD -- <files>
• git checkout -- <files>

Proposal

Compress to the same ok <result> pattern as rtk git add / rtk git commit:

• git checkout -b feat/x → ok feat/x (new)
• git checkout main → ok main (or ok main, 3 files updated)
• git checkout HEAD -- foo.ts bar.ts → ok 2 files restored
• On conflict / dirty tree: surface only the conflict/error, not the full status block

Why it matters

Plain git checkout on a branch with diff context can dump dozens of "M / D / ??" lines. Same shape as the existing rtk git pull compression — should be a small extension.