docs: document custom URL actions on Prompt component

[devin-ai-integration]

Summary

Updates the Prompt component documentation to reflect fern-platform#10416, which widened the actions prop to accept custom action objects alongside the built-in cursor, claude, and chatgpt shorthands.

Changes to fern/products/docs/pages/component-library/default-components/prompt.mdx:

• Updates the page intro and description frontmatter to mention custom URLs in addition to the built-in tools.
• Adds a new ### With a custom URL action variant section documenting:
  • The { label, url, icon? } object shape.
  • The {prompt} URL placeholder (URL-encoded prompt body) and the ?prompt= query-param fallback when the placeholder is omitted.
  • The http/https scheme restriction.
  • Mixing custom and built-in actions in the same array.
• Updates the actions <ParamField> to type (string | object)[] and adds nested <Indent> <ParamField>s for label, url, and icon.

Changes to fern/products/docs/pages/changelog/2026-05-01.mdx:

• Adds a ## Custom URL actions on the Prompt component section with a code sample and link to the new variant section.
• Adds the components tag.

Review & Testing Checklist for Human

• On the preview, confirm the new "With a custom URL action" example renders with two action buttons (built-in Cursor + the custom "Open in Plant ID" with a leaf icon) and that the dropdown / primary-button behavior matches the existing multi-action variant.
• Confirm the changelog entry's "Read the docs" button anchors to the new section (/learn/docs/writing-content/components/prompt#with-a-custom-url-action).
• Skim the updated actions Properties block to make sure the nested <Indent> <ParamField>s for label / url / icon read as sub-properties of actions and not as top-level props.

Notes

• Vale passes locally with 0 errors, 0 warnings, and 0 suggestions on the modified component page.
• The new variant uses plant-themed example content per the user's example-content guideline.

Link to Devin session: https://app.devin.ai/sessions/c5ce6253ec3244eeb9142f05e419475f
Requested by: @devalog

[devin-ai-integration]

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