feat(react): add shared markdown renderer for observer messages

[willwashburn]

Summary

• add a reusable MessageMarkdown component to @relaycast/react using react-markdown with GFM and soft line breaks
• export the renderer from the React SDK public API
• switch observer dashboard message and thread rendering to use the shared renderer
• add unit tests for markdown rendering and secure external-link attributes

Verification

• npm run -w @relaycast/react test
• npm run -w @relaycast/react build
• npm run -w @relaycast/react lint
• npm run -w @relaycast/observer-dashboard build

Notes

• no change to agent persona text rendering in this PR

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[github-actions]

Preview deployed!

Environment	URL
API	https://pr65-api.relaycast.dev
Health	https://pr65-api.relaycast.dev/health
Observer	https://pr65-observer.relaycast.dev

This preview shares the staging database and will be cleaned up when the PR is merged or closed.

Run E2E tests

npm run e2e -- https://pr65-api.relaycast.dev --ci

Open observer dashboard

https://pr65-observer.relaycast.dev

[willwashburn]

Addressed review feedback in 9ad8e2b:

• moved remarkPlugins to a module-level constant (REMARK_PLUGINS)
• hardened anchor rendering by spreading markdown props before explicit href/target/rel, and stripping node from DOM props
• expanded test coverage to assert internal links do not receive a rel attribute

I did not change the index.ts export grouping comment since it is stylistic and does not affect behavior or API shape.