Social connection layer

Likely out of scope for now. Each item below is a self-contained spike before any implementation.

Layer 1 — ActivityPub
- Spike: confirm ActivityPub plugin can be installed alongside Radical Socials with zero config. Each site gets a Fediverse identity, Mastodon/Pixelfed users can follow and reply
- If compatible: add ActivityPub to recommended plugins list and document the setup step

Layer 2 — Reader / following
- Spike: evaluate OpenSocial as a reader/following layer
- Spike: evaluate whether WP.com Reader can be extended, or whether a lightweight RSS-based follow list inside the plugin is sufficient
- Spike: Substack (their reader supports RSS); investigate whether Radical Socials sites can be followed from Substack and vice versa (likely free, no integration needed)
- private-reader.html theme template in theme is the intended surface for whichever approach wins

Layer 3 — Discovery / FYP
- Spike: tag-based discovery, surface other Radical Socials sites using the same social-tag terms via an opt-in site index

Start by evaluating whether the ActivityPub plugin works alongside Radical Socials with zero configuration, then review the recommended plugins list. Read theme/private-reader.html as the intended reader surface and investigate the proposed OpenSocial, WordPress.com Reader, RSS, Substack, and tag-index approaches. Done means the spikes are documented and a viable direction is selected for each layer.

Written by the indexing model from the issue text.