Add configurable git branching with three strategies:
- none (default): commit to current branch
- phase: create branch per phase (gsd/phase-{N}-{slug})
- milestone: create branch per milestone (gsd/{version}-{slug})
Changes:
- planning-config.md: add git.branching_strategy and templates
- execute-phase.md: handle branch creation based on strategy
- settings.md: add branching strategy to settings UI
- complete-milestone.md: handle branch merging at milestone end
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