Adds git.isolation: "none" so developers can use GSD's full planning/execution
methodology with all work happening directly on their current branch — no worktree,
no milestone branch, no merge step.
- Preference plumbing: "none" accepted/validated, getIsolationMode() helper
- Core bypass: all worktree/branch gates in auto.ts skip in none mode
- Peripheral: doctor skips worktree checks, system prompt omits worktree context
- Docs: git-strategy, configuration, auto-mode, troubleshooting, system prompt updated
- Tests: 11 new tests, 621/621 pass
