fix(parse): skip forge refs in right-to-left metadata scan

Problem: `parse.body()` scans tokens right-to-left and breaks on the
first non-metadata token. Forge refs like `gl:a/b#12` halted the scan,
preventing metadata tokens to their left (e.g. `due:tomorrow`) from
being parsed. Additionally, `diff.parse_buffer()` ignored
`metadata.priority` from `+!!` tokens and only used checkbox-derived
priority, and priority updates between two non-zero values were silently
skipped.

Solution: Recognize forge ref tokens via `forge.parse_ref()` during the
right-to-left scan and skip past them, re-appending them to the
description so `forge.find_refs()` still works. Prefer
`metadata.priority` over checkbox priority in `parse_buffer()`, and
simplify the priority update condition to catch all value changes.

lua/pending/diff.lua

@@ -63,7 +63,7 @@ function M.parse_buffer(lines)
           type = 'task',
           id = id and tonumber(id) or nil,
           description = description,
-          priority = priority,
+          priority = metadata.priority or priority,
           status = status,
           category = metadata.category or current_category or config.get().default_category,
           due = metadata.due,
@@ -146,10 +146,7 @@ function M.apply(lines, s, hidden_ids)
           task.category = entry.category
           changed = true
         end
-        if entry.priority == 0 and task.priority > 0 then
-          task.priority = 0
-          changed = true
-        elseif entry.priority > 0 and task.priority == 0 then
+        if entry.priority ~= task.priority then
           task.priority = entry.priority
           changed = true
         end