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.

doc/pending.txt

@@ -1501,6 +1501,7 @@ Configuration: ~
     vim.g.pending = {
       forge = {
         close = false,
+        validate = false,
         warn_missing_cli = true,
         github = {
           icon = '',
@@ -1525,6 +1526,10 @@ Top-level fields: ~
     {close}             (boolean, default: false) When true, tasks linked to
                         closed/merged remote issues are automatically marked
                         done on buffer open.
+    {validate}          (boolean, default: false) When true, new or changed
+                        forge refs are validated on `:w` by fetching metadata.
+                        Logs a warning if the ref is not found, auth fails, or
+                        the CLI is missing.
     {warn_missing_cli}  (boolean, default: true) When true, warns once per
                         forge per session if the CLI is missing or fails.