docs: Mark repository as deprecated in favour of aragon/app monorepo

[milosh86]

Description

Marks this repository as deprecated ahead of archiving. The Governance UI Kit now lives in the
aragon/app monorepo under
packages/gov-ui-kit. The package there is already at
2.11.4 with the same changelog, and it owns the npm publish and the Storybook deploy to uikit.aragon.org.

• README.md: add a deprecation warning that points to the monorepo and tells people to open issues and PRs there.
• AGENTS.md: add a deprecation note so agents stop making changes here; .github/copilot-instructions.md was regenerated with pnpm agents:sync.

Docs only, so no changeset is needed.

⚠️ Before merging: merging to main triggers storybook-publish.yml, which deploys this repo's Storybook to
uikit.aragon.org, the same domain the monorepo now deploys to. Disable Actions for this repo (or that workflow) before
merging so it doesn't overwrite the monorepo's Storybook.

Type of change

• Documentation only (non-breaking)

Developer Checklist:

• Made the corresponding changes to the documentation
• Selected the correct base branch
• Followed the code style guidelines of this project
• Reviewed that the files changed in GitHub’s UI reflect my intended changes
• Confirmed the pipeline checks are not failing

Review Checklist:

• Confirmed that changes follow the code style guidelines of this project

[jjavieralv]

LGTM