docs: codify agent quality guidelines

[leehack]

Summary

• Add AGENTS.md guidance for capability probes vs structural backend assumptions
• Document Web/WebGPU bridge runtime expectations, WASMFS durability caveats, and actionable unsupported errors
• Call out negative/version-skew regression coverage and docs/changelog alignment for public capability changes

Test Plan

• git diff --check HEAD~1..HEAD
• Markdown sanity check: no tabs/trailing whitespace

Notes

• Documentation-only agent guidance update; no runtime code changes.

[Copilot]

Pull request overview

This PR updates the repository’s agent guidance to better document how to reason about runtime capabilities (especially for web/WebGPU) and how to validate capability-dependent behavior.

Changes:

• Adds new guidance on preferring capability probes over structural/interface assumptions, and on failing unsupported paths with actionable diagnostics.
• Documents expectations for Web/WebGPU bridge versioning, durability caveats, and regression coverage (happy + negative/version-skew paths).
• Extends testing standards to explicitly require coverage for unsupported/version-skew capability paths and user-guiding error messages.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.23%. Comparing base (c2c0adc) to head (0843024).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
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  Coverage   78.23%   78.23%           
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  Files          75       75           
  Lines        9696     9696           
=======================================
  Hits         7586     7586           
  Misses       2110     2110           

Flag	Coverage Δ
unittests	78.23% <ø> (ø)

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.