feat(4.7): extract MCPToolHandlers into openspace/mcp/tool_handlers.py

Extract all 4 MCP tool handlers (execute_task, search_skills, fix_skill,
upload_skill) and their 12 helper functions from mcp_server.py into a new
openspace/mcp/tool_handlers.py module.

Changes:
- Create openspace/mcp/ package with __init__.py
- Create openspace/mcp/tool_handlers.py (799 lines) with:
  - 4 tool handlers + register_handlers(mcp) wiring function
  - All lifecycle helpers (_get_openspace, _get_store, etc.)
  - Upload metadata helpers (_write/_read_upload_meta)
  - Cloud import helpers (_cloud_search_and_import, _do_import_cloud_skill)
  - Result formatting (_format_task_result, _json_ok, _json_error)
- Slim mcp_server.py to 220 lines (was 992): bootstrap, FastMCP, server entry
- Update test imports: test_auto_import_disabled, test_integration,
  test_traceback_safety now reference openspace.mcp.tool_handlers
- Add 20 new tests in test_mcp_tool_handlers.py

All 1,376 tests pass, 117 skipped.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Brian Krafft

openspace/mcp/__init__.py

@@ -0,0 +1 @@
+"""OpenSpace MCP sub-package — decomposed from mcp_server.py (P4 Epics 4.7-4.9)."""