refactor: formatting fixes, config cleanup, health simplification

Problem: Formatter disagreements in `init.lua` and `gtasks.lua`,
stale `calendar` field in gcal config, and redundant health checks
for data directory existence.

Solution: Apply stylua formatting, remove `calendar` field from
`pending.GcalConfig`, drop data-dir and no-file health messages,
add `done`/`pending` to filter tab-completion candidates.

lua/pending/config.lua

@@ -7,7 +7,6 @@
 ---@field category string
 
 ---@class pending.GcalConfig
----@field calendar? string
 ---@field credentials_path? string
 ---@field client_id? string
 ---@field client_secret? string