feat: add <C-a> / <C-x> keymaps for priority increment/decrement (#114)

* feat(s3): create bucket interactively during auth when unconfigured

Problem: when a user runs `:Pending s3 auth` with no bucket configured,
auth succeeds but offers no way to create the bucket. The user must
manually run `aws s3api create-bucket` and update their config.

Solution: add `util.input()` coroutine-aware prompt wrapper and a
`create_bucket()` flow in `s3.lua` that prompts for bucket name and
region, handles the `us-east-1` LocationConstraint quirk, and logs a
config snippet on success. Called automatically from `auth()` when
`sync.s3.bucket` is absent.

* ci: typing

* feat(parse): add `parse_duration_to_days` for duration string conversion

Problem: The archive command accepted only a bare integer for days,
inconsistent with the `+Nd`/`+Nw`/`+Nm` duration syntax used elsewhere.

Solution: Add `parse_duration_to_days()` supporting `Nd`, `Nw`, `Nm`,
and bare integers. Returns nil on invalid input for caller error handling.

* feat(archive): duration syntax and confirmation prompt

Problem: `:Pending archive` accepted only a bare integer for days and
silently deleted tasks with no confirmation, risking accidental data loss.

Solution: Accept duration strings (`7d`, `3w`, `2m`) via
`parse.parse_duration_to_days()`, show a `vim.ui.input` confirmation
prompt before removing tasks, and skip the prompt when zero tasks match.

* feat: add `<C-a>` / `<C-x>` keymaps for priority increment/decrement

Problem: Priority could only be cycled with `g!` (0→1→2→3→0), with no
way to directly increment or decrement.

Solution: Add `adjust_priority()` with clamping at 0 and `max_priority`,
exposed as `increment_priority()` / `decrement_priority()` on `<C-a>` /
`<C-x>`. Includes `<Plug>` mappings and vimdoc.

* fix(s3): use parenthetical defaults in bucket creation prompts

Problem: `util.input` with `default` pre-filled the input field, and
the success message said "Add to your config" ambiguously.

Solution: Show defaults in prompt text as `(default)` instead of
pre-filling, and clarify the message to "Add to your pending.nvim
config".

* ci: format

lua/pending/config.lua

@@ -137,6 +137,8 @@ local defaults = {
     move_up = 'K',
     wip = 'gw',
     blocked = 'gb',
+    priority_up = '<C-a>',
+    priority_down = '<C-x>',
   },
   sync = {},
   icons = {