feat: add true agent-group incremental refresh

[Lling0000]

Summary

• replace path-only quick refresh with a committed-diff ImpactPlanner + independent ImpactVerifier loop
• update only approved Agent groups and related artifacts; unresolved plans preserve the active generation
• add generation-backed storage with atomic current.json promotion and resumable --failed-only staging
• require a clean worktree and a full baseline before quick refresh
• make rb-ask reminder-only instead of mutating knowledge automatically

Key behavior

• Git establishes committed change facts; RepoBrain decides affected groups
• candidate groups expand layer-by-layer through reverse dependencies
• Planner/Verifier conflicts retry for at most three rounds
• comment/format-only commits may advance with zero group execution
• quick refresh never falls back to a full refresh

Validation

• python3.12 -m compileall -q engine/repobrain_engine cli/src/rb_cli
• .venv/bin/pytest -q engine/tests cli/tests — 339 passed
• .venv/bin/python scripts/check_repo_contract.py
• git diff --check