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>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "autocad-mcp-pro"
-version = "1.1.0"
+version = "1.1.1"
 description = "Production-grade AutoCAD MCP server — COM API + ezdxf dual engine"
 readme = "README.md"
 requires-python = ">=3.11"