Add Copilot cloud-agent onboarding instructions for get.gov

[Copilot]

Ticket

Changes

• Onboarding baseline
  • Added .github/copilot-instructions.md with repo-specific guidance for first-time Copilot cloud agents.
• Execution guidance
  • Documented stack/context (Eleventy + USWDS), key paths (eleventy.config.mjs, config/buildAssets.js, content/layout directories), and high-value commands (npm install, npm run build, npm run dev, npm run pa11y).
• Known failure modes and workarounds
  • Captured that npm test is a placeholder and should not be used for validation in this repo.
  • Captured non-blocking Sass deprecation warning behavior during build and how to interpret it.

Context for reviewers

This PR onboards the repository to Copilot cloud agent by adding a single, focused instructions file to reduce first-run ambiguity and improve agent throughput. It codifies how to navigate the codebase, what to run, and how to handle the expected onboarding errors called out in the task.

Setup

sed -n '1,220p' .github/copilot-instructions.md

Code Review Verification Steps

As the original developer, I have

Satisfied acceptance criteria and met development standards

• Met the acceptance criteria, or will meet them in a subsequent PR
• Created/modified automated tests
• Update documentation in READMEs and/or onboarding guide

Ensured code standards are met (Original Developer)

• Error handling exists for unusual or missing values (N/A: documentation-only change)

Validated user-facing changes (if applicable)

• Tag @dotgov-designers in this PR's Reviewers for design review. If code is not user-facing, delete design reviewer checklist (N/A: non-user-facing)
• Verify new pages have been added to .pa11yci file so that they will be tested with our automated accessibility testing (N/A: no new site pages)
• Checked keyboard navigability (N/A: no UI change)
• Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI) (N/A: no UI change)

As a code reviewer, I have

Reviewed, tested, and left feedback about the changes

• Pulled this branch locally and tested it
• Verified code meets all checks above. Address any checks that are not satisfied
• Reviewed this code and left comments. Indicate if comments must be addressed before code is merged
• Checked that all code is adequately covered by tests

Validated user-facing changes as a developer

Note: Multiple code reviewers can share the checklists above, a second reviewer should not make a duplicate checklist. All checks should be checked before approving, even those labeled N/A.

• New pages have been added to .pa11yci file so that they will be tested with our automated accessibility testing
• Checked keyboard navigability
• Meets all designs and user flows provided by design/product
• Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI)

As a designer reviewer, I have

Verified that the changes match the design intention

• Checked that the design translated visually
• Checked behavior. Comment any found issues or broken flows.
• Checked different states (empty, one, some, error)
• Checked for landmarks, page heading structure, and links

Validated user-facing changes as a designer

• Checked keyboard navigability
• Tested general usability, landmarks, page header structure, and links with a screen reader (such as Voiceover or ANDI)
• Tested with multiple browsers (check off which ones were used)
  • Chrome
  • Microsoft Edge
  • FireFox
  • Safari

Screenshots

N/A (documentation-only change).