docs(mosaic): refine agent guidance from PR reviews

[austincalvelage]

Description

Turns recurring Mosaic PR-review findings into shared agent guidance for everyone working in the repository. Builds on #10115 with concrete implementation and verification guidance.

• Require reuse searches and abstractions that improve readability. Share stable business rules while allowing presentation to diverge.
• Clarify wrapper, controller, and view composition while preserving leaf controllers that receive effects through props.
• Add concrete checks for dialog reset and reopen behavior, pending actions, failed imports, explicit clock inputs and SSR hydration, and unknown-error causes and fallback messages.
• Keep one task-guide index in packages/mosaic/AGENTS.md. Correct styling and comment examples, qualify ARIA override behavior, and make the test example verify the active organization.

Only shared agent instructions and documentation change. Includes an empty changeset.

Checklist

• pnpm test runs as expected.
• pnpm build runs as expected.
• (If applicable) JSDoc comments have been added or updated for any package exports
• (If applicable) Documentation has been updated

Type of change

• 🐛 Bug fix
• 🌟 New feature
• 🔨 Breaking change
• 📖 Refactoring / dependency upgrade / documentation
• other:

[changeset-bot]

🦋 Changeset detected

Latest commit: 43ab22c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 0 packages

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