Add runner maintainer desired-state planner

[cbusillo]

Summary

• add a read-only runner maintainer desired-state planner contract
• wire work-graph runner-maintainer-plan for fixture-driven inventory/baseline planning
• separate runner_directory from allowed registration roots to preserve existing registration-root semantics
• distinguish policy readiness from supervised-maintainer capability readiness so blocked plans are not ambiguous
• document that recommendation decisions remain blocked by supervised_maintainer_required until the supervised host maintainer exists

Verification

• uv run python -m unittest tests.test_runner_lane_maintainer tests.test_runner_lane_control tests.test_runner_lane_registration
• uv run --extra dev ruff format --check control_plane/contracts/runner_lane_maintainer.py control_plane/cli_runner_lanes.py tests/test_runner_lane_maintainer.py
• uv run --extra dev ruff check control_plane/contracts/runner_lane_maintainer.py control_plane/cli_runner_lanes.py tests/test_runner_lane_maintainer.py
• uv run --extra dev mypy control_plane/contracts/runner_lane_maintainer.py control_plane/cli_runner_lanes.py tests/test_runner_lane_maintainer.py
• git diff --check