docs: add plugin pages and correct default-key claims
Wiki: drop the ADR references
The wiki is the operator's manual; a reader looking up how to point a
library at a folder does not want a decision record. Removed every
inline ADR citation and the pointer to docs/adr from Home, rewrapping
the sentences that carried them. CONTEXT.md stays linked from Home as
the glossary.
Claude-Session: https://claude.ai/code/session_01BKKpATzufuM2VQ79JFkTne
Wiki: linked servers
New Linked Servers page covering both sides of a link — the remote-role
user, library grants, the playback ceiling, the three reachability paths
(public HTTPS, reverse proxy, Tailscale machine-share), the one-time
invite, redemption, link states and the sync/re-key/unlink actions.
Cross-referenced from Home, the sidebar, First Run (users), Tailscale
(machine sharing) and HTTPS; OBELO_LINK_SYNC_INTERVAL added to
Configuration and a linked-servers section to Troubleshooting.
Claude-Session: https://claude.ai/code/session_01BKKpATzufuM2VQ79JFkTne
Wiki: setup, configuration, Docker, HTTPS, Tailscale, troubleshooting