feat(selection): add selection_get — read live viewport pickfirst selection (#1)

New COM-only Entity Query tool that reads AutoCAD's implied ("pickfirst")
selection set and returns the selected entity handles + EntityInfo, so the AI
can scope work (e.g. dimension_auto) to exactly what the user picked in the
viewport instead of the whole drawing. Resolves #1 — the layer-juggling
workaround is no longer needed.

- backends/base.py: abstract selection_get() returning a status dict
- backends/com_backend.py: reads ActiveDocument.PickfirstSelectionSet, surfaces
  the PICKFIRST sysvar, handles the empty-selection case
- backends/ezdxf_backend.py: headless stub (ok=False, empty handles, same shape)
- server.py: @mcp.tool registration (readOnlyHint), SECTION 5 3 -> 4 tools
- README + CHANGELOG + pyproject: 110 -> 111 tools, v1.1.0 -> v1.1.1

Verified live on AutoCAD 2027 (COM): selection_get correctly returned the
user's grip selection across two distinct picks; 318 tests pass, ruff-clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backends/ezdxf_backend.py

@@ -1466,6 +1466,19 @@ class EzdxfBackend(AutoCADBackend):
             ]
         return await self._async(_sync)
 
+    async def selection_get(self) -> dict:
+        # COM-only: headless ezdxf has no viewport / pickfirst grip selection.
+        # Return a not-supported result but keep the same shape (empty handles)
+        # so callers that iterate the selection do not KeyError.
+        return {
+            "ok": False,
+            "error": "selection_get not supported in ezdxf backend (no live AutoCAD viewport)",
+            "count": 0,
+            "handles": [],
+            "entities": [],
+            "pickfirst": None,
+        }
+
     # ── view / screenshot ──────────────────────────────────────────────────────
 
     async def view_zoom_extents(self) -> dict: