Enforce Dream skill ownership markers

[hamb1y]

Scope

Dream skill ownership enforcement.

Issues Fixed

• Fixes Security: Dream can overwrite user-created skills without ownership enforcement #4075: prevents Dream from overwriting existing user-created skills unless the skill is explicitly marked dream_managed: true.

Key Files

• nanobot/agent/memory.py
• tests/agent/test_dream.py

Validation

• uv run pytest tests/agent/test_dream.py::TestDreamRun::test_skill_write_tool_accepts_workspace_relative_skill_path tests/agent/test_dream.py::TestDreamRun::test_skill_write_tool_rejects_user_skill_without_dream_marker tests/agent/test_dream.py::TestDreamRun::test_skill_write_tool_allows_dream_managed_skill -q

[chengyongru]

related: #4003