[Docs] Update AGENTS.md with Sistent design guidance

[PARTH-TUSSLE]

Summary

Updates AGENTS.md to align the repository's UI guidance with its existing styling conventions and Sistent usage, while keeping the design guidance version-aware and avoiding duplicate documentation.

Details

• Styling: Removes the outdated Tailwind recommendation and keeps the guidance aligned with the repository's established styling patterns.
• UI & Sistent Guidance:
  • Directs contributors and AI agents to prefer existing UI primitives from @sistent/sistent before creating custom components.
  • Points Sistent-based UI work to node_modules/@sistent/sistent/DESIGN.md when available, or the Sistent release matching the version resolved in package-lock.json.
  • Keeps Sistent as the source of truth for applicable design tokens and component conventions without duplicating DESIGN.md.
  • Encourages reuse of existing Sistent theme/token values instead of hardcoding brand values.
• Component Generation: Updates guidance to check Sistent for existing primitives before creating new UI components.
• Cleanup: Removes the stale/unclosed Markdown code fence from AGENTS.md.

This PR fixes #8061

Notes for Reviewers

Signed commits

• [✅] Yes, I signed my commits.

Summary by CodeRabbit

• Documentation
  • Updated contribution guidance to prioritize themed styled-components and established styling conventions.
  • Added guidance for reusing Sistent UI components and design tokens to maintain visual consistency.
  • Clarified component-generation requirements, including checking for existing shared components before creating new primitives.
  • Prohibited introducing Tailwind CSS and discouraged hardcoded brand values when design tokens are available.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: ed4db673-d137-47dc-bcac-3cb1f8bd9816

📥 Commits

Reviewing files that changed from the base of the PR and between 51bdbbd and 8533cb9.

📒 Files selected for processing (1)

• AGENTS.md

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📝 Walkthrough

Walkthrough

AGENTS.md now defines styled-components and Sistent as the preferred UI guidance, prohibits new Tailwind CSS usage, updates component-generation instructions, and removes a stray closing code fence.

Changes

UI guidance

Layer / File(s)	Summary
Styling and component-generation guidance AGENTS.md	Styling guidance now prioritizes styled-components with theme tokens, permits established CSS Modules or Emotion usage, and prohibits introducing Tailwind CSS. The guidance requires reuse of @sistent/sistent components and checking Sistent before creating new UI primitives. Repository-specific UI must follow existing styling conventions. A stray closing code fence was removed.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to 8533c

The PR updates UI development guidance without indicating a production behavior change or merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the documentation file and the main change: adding Sistent design guidance. It is concise and directly related to the pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

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[suttonskate]

Great. I wonder if we can include a blurb about situations in which it might be time to break free from the mold and try a new UX paradigm to excite site visitors. After all, we do want Sistent to evolve an modernize.