Problem: sync backends were hardcoded in `SYNC_BACKENDS` list in
`init.lua`, auth routed directly through `oauth.google_client`, and
adding a non-OAuth backend required editing multiple files.
Solution: replace hardcoded list with `discover_backends()` that globs
`lua/pending/sync/*.lua` at runtime. Rewrite `M.auth()` to dispatch
to per-backend `auth()` methods with `vim.ui.select` fallback. Add
`lua/pending/sync/s3.lua` with push/pull/sync via AWS CLI, per-task
merge by `_s3_sync_id` (UUID), and `pending.S3Config` type.
Problem: several functions used inline `{...}` table types in their
`@param` and `@return` annotations, making them hard to read and
impossible to reference from other modules.
Solution: extract each into a named `---@class`: `pending.Metadata`,
`pending.TaskFields`, `pending.CompletionItem`, `pending.SystemResult`,
and `pending.OAuthClientOpts`.
