feat: add opt-in branching workflow

[AusafMo]

Summary

• add opt-in [branches] config and typed refs storage for Mongo/Postgres
• add branch create/list/delete, draft branch commits, branch diff/log, and PR create/list/show/close/merge
• expose branching through CLI, action layer, MCP tools, UI controls, docs, and the portable skill

Safety model

• branch commands, draft commits, branch diff, and PR creation do not mutate runtime
• cfg pr merge is the only branch/PR runtime mutation path
• PR merge refuses stale main heads and live drift; multi-record PR merge is blocked until adapter-level batch atomicity exists

Tests

• .venv/bin/python -m pytest tests/test_engine_safety.py -q twice: 28 passed
• .venv/bin/python -m pytest tests/test_config.py tests/test_ui_server.py tests/test_mcp_identity.py tests/test_engine_safety.py -q twice: 36 passed
• .venv/bin/python -m pytest -q twice before commit: 68 passed, 2 skipped
• .venv/bin/python -m pytest -q twice after rebase: 68 passed, 2 skipped
• CLI smoke: cfg --help, cfg branch --help, cfg pr --help, cfg --branch router-test log --help

Note

Browser connector reported no available browser backends in this session, so UI was covered by static/server tests rather than DevTools automation.