chore: update _instructions.md files for better copilot outputs

[nmerget]

Proposed changes

• fix: add some missing variables
• added further content to Copilot instructions for better Copilot outputs

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

🔭🐙🐈 Test this branch here: https://db-ux-design-system.github.io/core-web/review/chore-optimize-agent-instructions

[changeset-bot]

🦋 Changeset detected

Latest commit: cf7a9dd

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

This PR includes changesets to release 9 packages

Name	Type
@db-ux/core-foundations	Patch
@db-ux/core-components	Patch
@db-ux/agent-cli	Patch
@db-ux/ngx-core-components	Patch
@db-ux/react-core-components	Patch
@db-ux/wc-core-components	Patch
@db-ux/v-core-components	Patch
@db-ux/core-stylelint	Patch
@db-ux/core-migration	Patch

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