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/config.lua

@@ -41,6 +41,7 @@
 
 ---@class pending.ForgeConfig
 ---@field close? boolean
+---@field validate? boolean
 ---@field warn_missing_cli? boolean
 ---@field [string] pending.ForgeInstanceConfig
 
@@ -155,6 +156,7 @@ local defaults = {
   sync = {},
   forge = {
     close = false,
+    validate = false,
     warn_missing_cli = true,
     github = {
       icon = '',