When GSD auto-mode is running a planning phase, the planner subagent
could bypass GSD's state machine and artifact system. This adds a
shared state module and conflict check to block agents that overlap
with the active GSD phase.
- Add shared/gsd-phase-state.ts for cross-extension phase coordination
- Add conflicts_with frontmatter field to agent definitions
- Block conflicting agents with clear error directing to GSD workflow
- Tag planner agent with conflicts_with for plan/research phases
- 10 new tests for phase state and conflict parsing
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