docs: add issue scope split guardrails

[bigduu]

Summary

• add a 4-hour scope checkpoint and 12-hour hard split boundary for agent work
• define concrete split signals for subsystem count, acceptance slices, file count, and patch size
• require every adjacent review finding to be classified instead of automatically expanding the active Issue
• make cross-module parents tracking Issues with child-specific worktrees, branches, tests, and PRs
• preserve monolithic WIP only as extraction material instead of continuing sunk-cost polishing

This records the scope-control lesson from the Lotus #119 / Bamboo PR #869 audit.

Test Plan

• git diff --check
• verify the PR changes only AGENTS.md
• review Markdown placement under Multi-Agent Collaboration

Screenshots

Not applicable; documentation-only workflow guidance.