feat(diff): persist forge refs in store on write

Problem: forge refs parsed from buffer lines were discarded during
diff reconciliation and never stored in the JSON.

Solution: thread `forge_ref` through `parse_buffer` entries into
`diff.apply`, storing it in `task._extra._forge_ref` for both new
and existing tasks.

lua/pending/diff.lua

@@ -11,6 +11,7 @@ local parse = require('pending.parse')
 ---@field due? string
 ---@field rec? string
 ---@field rec_mode? string
+---@field forge_ref? pending.ForgeRef
 ---@field lnum integer
 
 ---@class pending.diff
@@ -65,6 +66,7 @@ function M.parse_buffer(lines)
           due = metadata.due,
           rec = metadata.rec,
           rec_mode = metadata.rec_mode,
+          forge_ref = metadata.forge_ref,
           lnum = i,
         })
       end
@@ -113,6 +115,7 @@ function M.apply(lines, s, hidden_ids)
           recur = entry.rec,
           recur_mode = entry.rec_mode,
           order = order_counter,
+          _extra = entry.forge_ref and { _forge_ref = entry.forge_ref } or nil,
         })
       else
         seen_ids[entry.id] = true
@@ -147,6 +150,13 @@ function M.apply(lines, s, hidden_ids)
             changed = true
           end
         end
+        if entry.forge_ref ~= nil then
+          if not task._extra then
+            task._extra = {}
+          end
+          task._extra._forge_ref = entry.forge_ref
+          changed = true
+        end
         if entry.status and task.status ~= entry.status then
           task.status = entry.status
           if entry.status == 'done' then
@@ -173,6 +183,7 @@ function M.apply(lines, s, hidden_ids)
         recur = entry.rec,
         recur_mode = entry.rec_mode,
         order = order_counter,
+        _extra = entry.forge_ref and { _forge_ref = entry.forge_ref } or nil,
       })
     end