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
* feat: add workflow mode system (solo/team) with /gsd mode command
Introduces a `mode` preference that bundles sensible defaults for solo
developers vs team workflows, replacing the need to manually configure
5-8 individual git preferences.
* fix: resolve TS2339 — use string narrowing for ctx.ui.select return type
