* refactor(sync): extract backend interface, adapt gcal module
Problem: :Pending sync hardcodes Google Calendar — M.sync() does
pcall(require, 'pending.sync.gcal') and calls gcal.sync() directly.
The config has a flat gcal field. This prevents adding new sync backends
without modifying init.lua.
Solution: Define a backend interface contract (name, auth, sync, health
fields), refactor :Pending sync to dispatch via require('pending.sync.'
.. backend_name), add sync table to config with legacy gcal migration,
rename gcal.authorize to gcal.auth, add gcal.health for checkhealth,
and add tab completion for backend names and actions.
* docs(sync): update vimdoc for backend interface
Problem: Vimdoc documents :Pending sync as a bare command that pushes
to Google Calendar, with no mention of backends or the sync table config.
Solution: Update :Pending sync section to show {backend} [{action}]
syntax with examples, add SYNC BACKENDS section documenting the interface
contract, update config example to use sync.gcal, document legacy gcal
migration, and update health check description.
* test(sync): add backend dispatch tests
Problem: No test coverage for sync dispatch logic, config migration,
or gcal module interface conformance.
Solution: Add spec/sync_spec.lua with tests for: bare sync errors,
empty backend errors, unknown backend errors, unknown action errors,
default-to-sync routing, explicit sync/auth routing, legacy gcal config
migration, explicit sync.gcal precedence, and gcal module interface
fields (name, auth, sync, health).
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