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

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/init.lua

@@ -489,9 +489,13 @@ function M._on_write(bufnr)
   if #stack > UNDO_MAX then
     table.remove(stack, 1)
   end
-  diff.apply(lines, s, hidden)
+  local new_refs = diff.apply(lines, s, hidden)
   M._recompute_counts()
   buffer.render(bufnr)
+  if new_refs and #new_refs > 0 then
+    local forge = require('pending.forge')
+    forge.validate_refs(new_refs)
+  end
 end
 
 ---@return nil