Remove fixed plugin count and generate the public overview from catalog - #2

Change

• Remove the fixed plugin count from public About positioning.
• Generate the README plugin index from the authoritative catalog and plugin manifest descriptions alongside the native marketplace. Future entries no longer require a manually rewritten overview table.
• Validate declared inventory and unique IDs rather than hardcoding the current plugin set. Reject a stale README projection.
• Document that titles/About must not freeze the current plugin count.

Verification

• Catalog generation and static validation pass.
• Isolated extension regression: add an additional plugin to a temporary catalog; generation and validation accept it and include it in the native marketplace and README automatically. Markdown table separators are escaped. A stale README row fails validation.
• No CLI pin, manifest, business behavior, Desktop routing or release changed.

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