Document public subreddit relaunch status and update stale share-post docs

[ripred]

Motivation

• Capture and communicate the current status of the public-facing subreddit rollout after multiple refactors so maintainers and operators know what is implemented vs what remains operational work.
• Align top-level documentation and the share-post examples with the code (share-post payloads, descriptor-based init routing, and sidebar widget automation) to avoid stale messaging.

Description

• Add docs/public-subreddit-status.md that documents the implemented public-subreddit architecture, supported share types, sidebar widget behavior, and a readiness checklist.
• Update README.md to mark the public-facing subreddit rollout as "under construction" and point to the dev subreddit as the active playtest.
• Update docs/share-post-examples/index.html copy to indicate the visual mocks are historical and that the share-post format is now implemented in-app.
• This PR only changes documentation and copy; no runtime game logic or server behavior was modified.

Testing

• Ran npm run type-check (tsc --build) to validate repository types, and it failed due to existing, repository-wide TypeScript issues unrelated to these docs-only changes.
• No other automated tests were added or required for this documentation alignment change.

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Codex Task