Add cfgit-agent coordination plugin

[AusafMo]

Summary

• add optional cfgit-agent package for multi-agent coordination over live datastore records
• implement sessions, leases, intents, idempotency, conflicts, events, JSON Patch validation/apply, and MCP wrappers
• add in-memory, Mongo, and Postgres agent-state adapters plus plugin-local [agent] config parsing
• add deterministic policy guardrails for claim scopes, deny paths, configured secret fields, and review-required paths
• route review-required agent patches to cfgit branch/PR flow without mutating runtime
• add an optional agent manager panel to the localhost cfgit UI
• document cfgit-agent in README, agent docs, skill, publishing docs, and the coordination spec

Verification

• .venv/bin/python -m pytest -q (100 passed, 4 skipped) twice
• .venv/bin/ruff check src tests plugins/cfg_impact/cfg_impact plugins/cfg_agent/cfg_agent
• git diff --check
• python -m build + twine check for cfgit, cfgit-impact, and cfgit-agent
• private-string scan across new public files

Notes

• cfgit-agent is in-repo only for now; docs keep it out of the PyPI publish workflow until its project/trusted publisher is configured
• no remote or cross-project Mongo write operations were used