Fix #414: select, move and delete ruler guides with the selection tools

Guides dragged out of the rulers could not be touched again: no tool hit-tested
them, so a click on a guide went to the art under it.

- The Selection, Direct Selection and Group Selection tools pick a ruler guide
  within the selection tolerance (over the art; Direct Selection takes an
  anchor on it first) through a shared tools::rulerguide::GuideEdit. A drag
  moves the selected guides in one undo step (Alt copies them, Shift snaps the
  dragged one to the ruler's ticks, otherwise it snaps to pixels, the grid or,
  with Smart Guides, the art's edges, centres and anchors); dropped off the
  canvas onto its ruler, the guide is deleted. Hidden or locked guides can't be
  picked; the pointer shows which way a guide moves.
- Selection::guides holds the selected guides (on their own, like slices); the
  canvas draws them in the selection colour. Delete/Backspace (edit.clear)
  delete them, the arrow keys (object.nudge) nudge them, Lock Guides and Hide
  Guides deselect them.
- Commands for agents: guide.list, guide.select {indexes, toggle?},
  guide.move {dx?, dy?, copy?} (besides {index, pos}) and guide.remove {index?};
  guide positions out of range are refused. Documented in docs/mcp.md.
- Shift while dragging a new guide out of a ruler snaps it to the ticks too
  (Unit::ruler_step, shared with the rulers' drawing).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/mcp.md

@@ -549,6 +549,29 @@ names must stay unique. In the desktop app the saved selections are listed at th
 (`select.recall1` … `select.recall25`). A file's saved selections are checked when it opens: past 25, blank or
 repeated names, and ids the document doesn't have are dropped.
 
+## Ruler guides
+
+Ruler guides are numbered in the order they were made. `guide.add {vertical, pos}` makes one (the x of a vertical
+guide, the y of a horizontal one, in points) → `{index}`; `guide.list` → `[{index, vertical, pos, selected}…]`.
+`guide.select {indexes: [index…], toggle?}` selects guides on their own (the art is deselected; `toggle` adds or
+removes them) → `{selected}`. `guide.move {index, pos}` puts one guide somewhere, `guide.move {dx?, dy?, copy?}`
+moves the selected ones (vertical guides by `dx`, horizontal ones by `dy`; `copy` leaves them and selects the
+moved copies), and `guide.remove {index?}` deletes one guide or the selected ones → `{count}`; each is one undo
+step. With guides selected, `edit.clear` (Delete) deletes them and `object.nudge` (the arrow keys) nudges them.
+View › Guides › Lock Guides (`view.guides.lock`) deselects them and refuses these commands until unlocked.
+
+The Selection, Direct Selection and Group Selection tools pick a guide within the selection tolerance, over the art
+(an anchor on it comes first with Direct Selection), and drag the selected guides: `mods.alt` copies them,
+`mods.shift` snaps the dragged guide to the ruler's ticks, and otherwise it snaps to whole pixels, the grid or, with
+Smart Guides, the art's edges, centres and anchors. In the desktop app a guide dropped off the canvas onto its
+ruler is deleted, and hidden guides (View › Guides › Hide Guides) can't be picked.
+
+```json
+{"name":"run_command","arguments":{"command":"guide.add","params":{"vertical":true,"pos":100}}}
+{"name":"pointer_gesture","arguments":{"tool":"selection","events":[{"kind":"down","x":100,"y":50},{"kind":"drag","x":140,"y":50},{"kind":"up","x":140,"y":50}]}}
+{"name":"run_command","arguments":{"command":"guide.list","params":{}}}
+```
+
 ## Graphic styles
 
 A graphic style holds an appearance (fills, strokes, effects) plus opacity, blend mode, isolate and knockout.