docs(workflow): 补充保持 main 线性的交付规则

[iamsyc]

What Changed

• added a delivery-flow rule to define what "keep main linear" means in this repository
• documented the required branch update step before PR open or merge
• documented the merge strategy preference to avoid merge commits on main

Why

• the previous wording was implicit and could be interpreted inconsistently
• this makes future PR delivery behavior precise and repeatable

Validation

• inspected the AGENTS.md diff
• ran git diff --check -- AGENTS.md

[github-actions]

CI Summary

Run: Open

Check	Status	Details
Change Scope	non_code	non-code pull request fast-path
Unit Tests	skipped	skipped; non-code pull request
UI Smoke Tests (matrix: baseline + permissionDenied + rebuildFailed)	skipped	skipped; non-code pull request
Release Build Check (Release arch smoke: arm64 + x86_64)	skipped	skipped; non-code pull request
CI Gate	success	required; non-code PR fast-path

Gate rule: 本次 PR 仅包含非代码变更，已跳过 Unit Tests / UI Smoke Tests / Release Build Check，ci-gate 直接通过。