feat(P4/4.3): extract ExecutionEngine from OpenSpace

Extract task execution lifecycle, two-phase skill orchestration,
and post-execution analysis into ExecutionEngine class.

Moved methods:
  - execute() → ExecutionEngine.execute()
  - _maybe_analyze_execution() → ExecutionEngine._maybe_analyze_execution()
  - _maybe_evolve_quality() → ExecutionEngine._maybe_evolve_quality()
  - Workspace resolution → ExecutionEngine._resolve_workspace()
  - Workspace cleanup → ExecutionEngine._cleanup_workspace()

Also extracted _execute_skill_first() and _log_result() as internal helpers.

OpenSpace.execute() is now a thin delegate.
ExecutionEngine is created at end of initialize() with all dependencies.

tool_layer.py: 735 → 442 lines (-40%)
execution_engine.py: 459 lines (new)
20 new tests in test_execution_engine.py
All 1,356 existing tests pass unchanged.

Epic 4.2 (SkillSelector) skipped — already extracted in 4.1.

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

Author: Brian Krafft