feat(sync): backend interface + CLI refactor (#42)

* 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).

plugin/pending.lua

@@ -171,6 +171,34 @@ end, {
     if cmd_line:match('^Pending%s+edit') then
       return complete_edit(arg_lead, cmd_line)
     end
+    if cmd_line:match('^Pending%s+sync') then
+      local after_sync = cmd_line:match('^Pending%s+sync%s+(.*)')
+      if not after_sync then
+        return {}
+      end
+      local parts = {}
+      for part in after_sync:gmatch('%S+') do
+        table.insert(parts, part)
+      end
+      local trailing_space = after_sync:match('%s$')
+      if #parts == 0 or (#parts == 1 and not trailing_space) then
+        local backends = {}
+        local pattern = vim.fn.globpath(vim.o.runtimepath, 'lua/pending/sync/*.lua', false, true)
+        for _, path in ipairs(pattern) do
+          local name = vim.fn.fnamemodify(path, ':t:r')
+          table.insert(backends, name)
+        end
+        table.sort(backends)
+        return filter_candidates(arg_lead, backends)
+      end
+      if #parts == 1 and trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      if #parts >= 2 and not trailing_space then
+        return filter_candidates(arg_lead, { 'auth', 'sync' })
+      end
+      return {}
+    end
     return {}
   end,
 })