docs: explain checkpoints, /rollback, and git worktrees

[teknium1]

Salvage of PR #1162 by @aydnOktay — cherry-picked docs commit onto current main with authorship preserved.

Adds two new user guide pages:

• Checkpoints and /rollback — how shadow git repos work, listing/restoring snapshots, safety guards
• Git Worktrees — running multiple agents on the same repo with isolated checkouts

Changes from original PR:

• Dropped the cli.py change (removed _rich_text_from_ansi() — functional regression unrelated to docs)
• Fixed the hermes -w description: it auto-creates a disposable worktree, doesn't take a path argument