docs: add user-facing documentation for state backends (git-notes, orphan-branch) #995

Squad has git-notes and orphan-branch state backends (shipped in PR #830). Fully implemented and tested (30+ tests) but no user-facing docs exist.

Needs:

1. What state backends are and why (state survives branch switches, no diff pollution)
2. Three options: worktree (default), git-notes, orphan
3. How to configure: .squad/config.json or --state-backend CLI flag
4. Priority chain: CLI > config > default
5. Fallback behavior

Where: README SDK section, docs/features/state-backends.md, CLI help text.

Related: state-backend.ts, test/state-backend.test.ts, PR #810 (closed, folded into #830)