Escape dynamic values in the customizations index

[Sriraj (sricursion)]

Fixes #327467

What changed

• XML-escape the customizations-index text fields requested in review: name, description, applyTo, and argumentHint.
• Apply the same producer-side fix in both the core and built-in Copilot implementations, including generated nested-AGENTS.md descriptions and the truncated skill-name fallback.
• Add regressions showing that adversarial closing-tag payloads stay encoded text and cannot create additional instruction, skill, or agent records.
• Keep file-path serialization and all consumers unchanged; this patch adds no format marker, decoding, parser, authorization, or cache behavior.

Validation

• npm run typecheck-client
• npm run transpile-client
• Core computeAutomaticInstructions.test.ts: 64 passing
• npm run eslint
• npm run valid-layers-check
• npm run define-class-fields-check
• Copilot npm run typecheck
• Copilot npm run lint
• Copilot npm run compile
• Copilot focused automaticInstructionsCollector.spec.ts: 32 passing
• Copilot full npm run test:unit: 9,553 passing, 129 skipped
• git diff --check

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Paul (pwang347)]

Hi Sriraj (@sricursion), thank you for opening a PR!

Could we please scope this down to just escaping the input text fields (ie., wrapping name, description, applyTo, argumentHint)? We should try to keep the changes here simple. Thanks!

[Sriraj (sricursion)]

Thanks for the guidance Paul (@pwang347) . I’ve scoped the PR down accordingly. It now only escapes name, description, applyTo, and argumentHint in both customizations-index producers.

I removed the format-marker, decoder, parser, cache, authorization, and file-path changes. Focused regression tests cover structural-tag injection through the retained text fields, and the typechecks, compilation, lint, focused tests, and full Copilot unit suite pass. The PR description has also been updated to reflect the narrower change.

[Paul (pwang347)]

Looks good to me. Thank you for contributing!