chore: improve AGENTS.md to enforce upstream workflow and project scripts

[xiaoxiaojx]

Summary

The previous AGENTS.md passively pointed to the upstream webpack AGENTS.md ("see ..."), which AI agents tended to skip — missing key rules like test-first for bug fixes and commit identity for CLA. It also said "refer to scripts in package.json" without being explicit enough, leading agents to use npx instead of project scripts.

This change makes the instructions actionable:

1. MUST fetch and read the upstream AGENTS.md before starting any task
2. MUST use project scripts from package.json, never npx

This file is designed to be copied across all webpack sub-repos.

What kind of change does this PR introduce?

chore

Did you add tests for your changes?

N/A — documentation only.

Does this PR introduce a breaking change?

No.

Use of AI

This PR was authored with the assistance of Claude Code (Claude Opus 4.6), based on workflow issues identified during a prior bug-fix session.

[changeset-bot]

⚠️ No Changeset found

Latest commit: dc1378d

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.56%. Comparing base (292aee5) to head (dc1378d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
+ Coverage   96.54%   96.56%   +0.01%     
==========================================
  Files          50       50              
  Lines        2927     2937      +10     
  Branches      922      928       +6     
==========================================
+ Hits         2826     2836      +10     
  Misses         85       85              
  Partials       16       16              

Flag	Coverage Δ
integration	96.56% <ø> (+0.01%)	⬆️

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[codspeed-hq]

Merging this PR will improve performance by ×55

⚡ 3 improved benchmarks
✅ 135 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	tsconfig-paths: 5 path prefixes (warm)	96.6 KB	1.8 KB	×55
⚡	Memory	resolve-to-context: directory resolve (warm)	96.6 KB	1.8 KB	×54
⚡	Memory	array-alias: @ -> [preferred, fallback] (warm)	1.8 KB	1.5 KB	+16.92%

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_{Comparing chore/improve-agents-md (dc1378d) with main (f28d8b9)}