Fix WordPress.org plugin review issues

[pento]

Summary

Addresses the three issues raised during WordPress.org plugin review:

• Source code documentation: Added a == Development == section to readme.txt linking to the GitHub repo and documenting the build tooling (@wordpress/scripts), satisfying the human-readable code guideline.
• Transient key prefix: Renamed the transient cache key prefix from the generic og_embed_ to opengraph_fallback_embed_ and extracted it as a CACHE_KEY_PREFIX class constant. Old transients expire naturally within 7 days.
• Inline <style> documentation: Expanded the code comment explaining why inline styles are required — the block editor renders oEmbed "rich" responses in a sandboxed iframe (wp.components.SandBox), and wp_enqueue_style() cannot reach it. This is a false positive from the reviewer and will need to be explained in the reply.

Test plan

• Run npm test to verify all PHPUnit tests pass with the new transient prefix
• Verify the Development section renders correctly on WordPress.org (after deployment)
• Confirm the inline style comment accurately describes the SandBox constraint

[Copilot]

Pull request overview

This PR updates the plugin to address WordPress.org review feedback by improving documentation, tightening transient cache key naming, and clarifying why inline styles are necessary for editor embeds.

Changes:

• Add a == Development == section to readme.txt linking to the GitHub repo and describing build tooling/source layout.
• Rename the transient cache key prefix to a plugin-specific value and extract it into a CACHE_KEY_PREFIX constant; update tests accordingly.
• Expand the inline-style comment explaining the block editor sandboxed iframe constraint for “rich” oEmbed responses.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
class-opengraph-fallback-embed.php	Introduces CACHE_KEY_PREFIX, updates transient key generation, and expands the inline-style rationale comment.
readme.txt	Adds Development section and a new changelog entry documenting the review-driven changes.
tests/ParseOgTagsTest.php	Updates transient cleanup to use the new cache key prefix constant.
tests/MaybeEmbedTest.php	Updates transient cleanup to use the new cache key prefix constant.
tests/EmbedBlockFallbackTest.php	Updates transient cleanup for both URLs to use the new cache key prefix constant.

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