Agent: catalog and garden tools, and a ready date on every describe group

- update_seed_lot / delete_seed_lot: correct or drop a recorded purchase
  ("it was three packets, not two"); the plant a lot is for stays fixed.
- delete_plant: remove a duplicate from the user's catalog. The service
  already refuses while plantings (past seasons included) or a lot reference
  it; the tool turns that sentinel into words the model can pass on, and
  tells it not to clear those references to get its way.
- create_garden: a new place, with the service's defaults; the prompt says a
  plan is still a copy_garden.
- describe_garden groups carry readyAround — planting date plus days to
  maturity for the plops still in the ground — so "what can I pick this
  week?" is a lookup rather than arithmetic the model got wrong live.

Co-Authored-By: Claude Fable 5 <[email protected]>

internal/agent/runtime.go

@@ -296,8 +296,13 @@ How to work:
   change that planted the beets, not pulling them out today; do not re-create what you can
   revert. A change already marked undone stays undone.
 - To correct a record rather than change the garden — a planting date, a plant count, a journal
-  entry's text or date, the garden's notes — use update_planting, update_journal_entry and
-  update_garden instead of removing and re-adding.
+  entry's text or date, the garden's notes, a seed lot — use update_planting, update_journal_entry,
+  update_garden and update_seed_lot instead of removing and re-adding.
+- "What can I pick soon?": each group in describe_garden carries readyAround, its planting date
+  plus the plant's days to maturity. Compare that with today rather than doing the sums yourself;
+  a group without it is a plant the catalog has no days for.
+- A new garden (another place, not a plan) is create_garden; it opens from the gardens list, and
+  this conversation stays with the garden it started in.
 - When the gardener tells you something worth keeping about the place — their zone, usual
   frost dates, soil, a standing preference — add it to the garden's notes with update_garden
   (keeping what is already there), and say you did. You will see those notes in every later