Rebase managed features safely

[crypdick]

Summary

• add a host-bound rebase_managed_feature lifecycle tool for manifest-managed branches
• verify and persist the remote default base so conflict recovery can continue safely
• document the bound rebase contract and cover host, IPC, and agent-tool seams

Checks

• uv run pytest tests/test_managed_feature_resolution.py tests/test_ipc_managed_feature_cop_gate.py tests/test_actions.py -q
• uv run ruff check on changed host, agent, and test files
• uv run mkdocs build --strict

prek was attempted but stalled before running child hooks in this host environment.

[crypdick]

Superseded by #107, which contains the completed implementation and passed all required checks.