feat(sql): schema-aware completion for db blocks via the LSP

[gandarfh]

Phase 3 of SQL support: the language server now offers context-aware SQL
completion inside ```db-* blocks (tables/columns from the app's cached
introspection), so completion is shared (desktop today, TUI later) instead
of reimplemented per surface.

• Sql.complete ~tables ~text ~offset (pure): after FROM/JOIN/INTO/UPDATE ->
  table names; after a table. qualifier -> that table's columns; otherwise
  columns + tables; partial word matched case-insensitively; column detail
  carries the data type. Lexical context via the CST (last_keyword_before)
  so a half-typed tail still classifies.
• Sql_schema_store.tables_for ~connection_id: reads the schema_cache
  table (connection id = the db-<conn> fence suffix), grouping rows into
  Sql.tables; degrades to [] on no-db / read error / cold cache.
• on_completion: outside a {{ref}}, a db-* block falls back to SQL
  completion.
• 6 pure completion tests; 146 pass, 0 fail.

No desktop change yet (it keeps its client-side completion); retiring
cm-db-schema-complete.ts lands with a later release.