fix(sync): replace cryptic sigil counters with readable output

Problem: sync summaries used unexplained sigils (`+/-/~` and `!`) that
conveyed no meaning, mixed symbol and prose formats across operations,
and `gcal push` silently swallowed failures with no aggregate counter.

Solution: replace all summary `log.info` calls with a shared
`fmt_counts` helper that formats `N label` pairs separated by ` | `,
suppresses zero counts, and falls back to "nothing to do". Add a
`failed` counter to `gcal.push` to surface errors previously only
emitted as individual warnings.

lua/pending/sync/gtasks.lua

@@ -195,6 +195,21 @@ local function unlink_remote(task, now_ts)
   task.modified = now_ts
 end
 
+---@param parts {[1]: integer, [2]: string}[]
+---@return string
+local function fmt_counts(parts)
+  local items = {}
+  for _, p in ipairs(parts) do
+    if p[1] > 0 then
+      table.insert(items, p[1] .. ' ' .. p[2])
+    end
+  end
+  if #items == 0 then
+    return 'nothing to do'
+  end
+  return table.concat(items, ' | ')
+end
+
 ---@param task pending.Task
 ---@return table
 local function task_to_gtask(task)
@@ -451,7 +466,13 @@ function M.push()
       buffer.render(buffer.bufnr())
     end
     log.info(
-      string.format('Google Tasks pushed — +%d ~%d -%d !%d', created, updated, deleted, failed)
+      'gtasks push: '
+        .. fmt_counts({
+          { created, 'added' },
+          { updated, 'updated' },
+          { deleted, 'deleted' },
+          { failed, 'failed' },
+        })
     )
   end)
 end
@@ -475,13 +496,13 @@ function M.pull()
       buffer.render(buffer.bufnr())
     end
     log.info(
-      string.format(
-        'Google Tasks pulled — +%d ~%d !%d, unlinked: %d',
-        created,
-        updated,
-        failed,
-        unlinked
-      )
+      'gtasks pull: '
+        .. fmt_counts({
+          { created, 'added' },
+          { updated, 'updated' },
+          { unlinked, 'unlinked' },
+          { failed, 'failed' },
+        })
     )
   end)
 end
@@ -507,17 +528,20 @@ function M.sync()
       buffer.render(buffer.bufnr())
     end
     log.info(
-      string.format(
-        'Google Tasks synced — push: +%d ~%d -%d !%d, pull: +%d ~%d !%d, unlinked: %d',
-        pushed_create,
-        pushed_update,
-        pushed_delete,
-        pushed_failed,
-        pulled_create,
-        pulled_update,
-        pulled_failed,
-        unlinked
-      )
+      'gtasks sync — push: '
+        .. fmt_counts({
+          { pushed_create, 'added' },
+          { pushed_update, 'updated' },
+          { pushed_delete, 'deleted' },
+          { pushed_failed, 'failed' },
+        })
+        .. '  pull: '
+        .. fmt_counts({
+          { pulled_create, 'added' },
+          { pulled_update, 'updated' },
+          { unlinked, 'unlinked' },
+          { pulled_failed, 'failed' },
+        })
     )
   end)
 end