Enhance AGENTS.md guidance for typed tests and ESLint suppressions #1419

Summary

Enhance AGENTS.md so agents do not add blanket ESLint suppression entries when they add or modify tests with many @typescript-eslint/no-explicit-any violations.

Rationale

AGENTS.md already states that suppression counts must not increase and that new code must use typed test doubles or unknown guards instead of any. The guidance needs a concrete workflow for large test files. This will make the rule easier to apply during implementation and review.

Affected area

• AGENTS.md
• Test files that use @typescript-eslint/no-explicit-any suppression counts in src/eslint-suppressions.json

Required changes

• Add a concise test-typing checklist for new and substantially changed test files.
• Require typed structural doubles, typed mock helpers, or unknown plus type guards before a suppression is considered.
• Prohibit adding a new per-file no-explicit-any suppression entry for a new test file.
• Require a narrow ESLint command with --prune-suppressions after test typing changes.
• State that an unavoidable exception requires a local explanation and maintainer approval.

Acceptance criteria

• AGENTS.md contains the new checklist and exception process.
• The text does not conflict with the existing ESLint Suppressions section.
• The change does not increase any count in src/eslint-suppressions.json.

References

• Pull request: fix(webview): add durable per-view state base #977
• Review comment: fix(webview): add durable per-view state base #977 (comment)
• Requested by: @easonLiangWorldedtech