Address #127 review: quote the garden name, validate rectangles, require plantId

- The plan-name line of the system prompt interpolates the garden's name
  with %q like the rest of the prompt: any editor can rename a garden, and a
  name with a newline in it must not read as an instruction.
- fill_region refuses an inverted rectangle with its corners named, and a
  rectangle that misses the bed (or only touches its edge) is an error from
  the service rather than a successful fill of nothing.
- remove_plantings requires plantId; omitted it would remove plant 0 and
  report success.
- historyEntry.Undo → UndoOf (it holds the reverted change set's id).
- remove_planting's description names list_plantings as an id source.
- RemovePlanting takes the removal date itself; the dateless wrapper had no
  callers left.

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

internal/agent/runtime.go

@@ -261,7 +261,7 @@ How to work:
 - fill_region in grid mode lays out individual plants at true spacing, which is what "so I can
   plant from it" means; clump mode is a quick sketch. For an area no compass name describes (a
   middle third, a strip along one edge) give fill_region a rectangle instead of placing plops by hand.
-- A garden named "%s — <year>" is this garden's plan for that year; copy_garden with that name
+- A garden named %s is this garden's plan for that year; copy_garden with that name
   makes one. Never use a different real garden as a scratch space.
 - When a tool refuses (for example, the user only has view access to this garden), explain what
   happened in plain words. Do not retry it.
@@ -280,5 +280,8 @@ How to behave:
   observation — not to narrate your own planting.
 - The gardener is watching the canvas. When you are done, say briefly what you changed and where
   to look; if you changed nothing, say that too.`,
-		g.Name, g.ID, size, today, units, g.Name)
+		// %q throughout for the garden's name: any editor can rename a garden, and
+		// a name is data, not prompt — quoting keeps a newline or a stray quote
+		// in it from reading as a new instruction.
+		g.Name, g.ID, size, today, units, fmt.Sprintf("%q", g.Name+" — <year>"))
 }