docs(plan): plant hours weekly log implementation plan
Bite-sized TDD task decomposition covering 13 tasks:
1. PocketBase schema (vehicles.unit_type + plant_hours_log)
2. Type updates (UnitType, PlantHoursLogRecord, BatchType)
3. Shared import-parse helpers + parseTime
4. Per-row validator + derived fields
5. Template generator + seed script
6. GET /api/plant-hours/template
7. Exception-report CSV builder + route
8. POST /api/plant-hours/import
9. DELETE /api/plant-hours/[id]
10. PATCH /api/vehicles/[id] accepts unit_type
11. ImportPlantHoursButton component
12. /dashboard/plant page extension
13. End-to-end smoke test
Each task has files, failing tests, implementation code, run commands,
and commit step. Follows the approved design spec.
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