docs: rewrite readme.txt and add readme writing guide

[ifahimreza]

What

Rewrites readme.txt in plain, short sentences, following the Elementor readme's layout. Adds a "WordPress plugin readme writing" guide to CLAUDE.md (and so to AGENTS.md, which links to it).

Why

The old readme was long, repeated itself and read like generated copy. The new guide keeps future edits to the same standard.

How

• Each feature line reads "Feature name: What the user can do."
• Free and Pro features have separate sections. The API key, OAuth and account requirements are stated.
• External services and privacy facts keep the same substance, including the precise update-check wording.
• The 1.0.0 changelog is cut to "Initial release." The full notes stay in git history.
• Version, Tested up to, tags and links are unchanged.

Open questions

• The Pro section lists Yoast, Rank Math, AIOSEO and WooCommerce tools. They exist in saddle-pro code but aren't in Pro's own readme yet.
• The ChatGPT FAQ says "Yes" and doesn't mention that Plus and Pro accounts are read-only for custom connectors.

Testing

• Short description is 126 characters
• No em dashes, no sentence of 20 words or more

This ships to WordPress.org on the next release, so it needs your review before merge.

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