- Fix build warnings: add #[allow(dead_code)] to unused deserialization fields
- Fix plan approval gate bug: block file changes when no plan exists (not just
when plan exists but is unapproved)
- Add "Create a plan: " prefix to first user message in plan mode
- Add prepare_plan_mode_input() helper function for testability
- Reset is_first_plan_message flag when entering plan mode via /plan command
- Add tests for approval gate (no plan + no changes, no plan + changes)
- Add tests for prepare_plan_mode_input (happy, negative, boundary cases)
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