feat(forge): add validate option for forge ref validation on write (#138)

Problem: Typos in forge refs like `gh:user/repo#42` silently persist —
there's no feedback when a ref points to a nonexistent issue.

Solution: Add `forge.validate` config option. When enabled, `diff.apply()`
returns new/changed `ForgeRef[]` and `forge.validate_refs()` fetches
metadata for each, logging specific warnings for not-found, auth, or
CLI-missing errors.

lua/pending/diff.lua

@@ -82,14 +82,26 @@ function M.parse_buffer(lines)
   return result
 end
 
+---@param a? pending.ForgeRef
+---@param b? pending.ForgeRef
+---@return boolean
+local function refs_equal(a, b)
+  if not a or not b then
+    return false
+  end
+  return a.forge == b.forge and a.owner == b.owner
+    and a.repo == b.repo and a.number == b.number
+end
+
 ---@param lines string[]
 ---@param s pending.Store
 ---@param hidden_ids? table<integer, true>
----@return nil
+---@return pending.ForgeRef[]
 function M.apply(lines, s, hidden_ids)
   local parsed = M.parse_buffer(lines)
   local now = timestamp()
   local data = s:data()
+  local new_refs = {} ---@type pending.ForgeRef[]
 
   local old_by_id = {}
   for _, task in ipairs(data.tasks) do
@@ -120,6 +132,9 @@ function M.apply(lines, s, hidden_ids)
           order = order_counter,
           _extra = entry.forge_ref and { _forge_ref = entry.forge_ref } or nil,
         })
+        if entry.forge_ref then
+          table.insert(new_refs, entry.forge_ref)
+        end
       else
         seen_ids[entry.id] = true
         local task = old_by_id[entry.id]
@@ -154,6 +169,10 @@ function M.apply(lines, s, hidden_ids)
           end
         end
         if entry.forge_ref ~= nil then
+          local old_ref = task._extra and task._extra._forge_ref or nil
+          if not refs_equal(old_ref, entry.forge_ref) then
+            table.insert(new_refs, entry.forge_ref)
+          end
           if not task._extra then
             task._extra = {}
           end
@@ -188,6 +207,9 @@ function M.apply(lines, s, hidden_ids)
         order = order_counter,
         _extra = entry.forge_ref and { _forge_ref = entry.forge_ref } or nil,
       })
+      if entry.forge_ref then
+        table.insert(new_refs, entry.forge_ref)
+      end
     end
 
     ::continue::
@@ -202,6 +224,7 @@ function M.apply(lines, s, hidden_ids)
   end
 
   s:save()
+  return new_refs
 end
 
 return M