refactor(config): remove legacy gcal top-level config key

Problem: the gcal migration shim silently accepted vim.g.pending = { gcal
= {...} } and copied it to sync.gcal, adding complexity and a deprecated
API surface.

Solution: remove the migration block in config.get(), drop the cfg.gcal
fallback in gcal_config(), delete the two migration tests, and clean up
the vimdoc references. Callers must now use sync.gcal directly.

lua/pending/sync/gcal.lua

@@ -23,7 +23,7 @@ local SCOPE = 'https://www.googleapis.com/auth/calendar'
 ---@return table<string, any>
 local function gcal_config()
   local cfg = config.get()
-  return (cfg.sync and cfg.sync.gcal) or cfg.gcal or {}
+  return (cfg.sync and cfg.sync.gcal) or {}
 end
 
 ---@return string