upgrade-forks: fix relative fork_dir after cd into fork (cd-back bug)

The script cd's into the fork dir for the clean-tree check (line ~202)
and never returns to PROJECT_ROOT. Every subsequent relative
'cd "$fork_dir"' subshell then fails with 'No such file or directory',
making single-fork and multi-fork runs always FAIL at the upstream-ref
resolution step. Make fork_dir absolute at assignment so all subshell
cd/git calls work regardless of current directory.

Found while attempting the base fork sync (verify-fork-functions gate).

local/scripts/upgrade-forks.sh

@@ -146,7 +146,7 @@ declare -a FAILED_FORKS=()
 declare -a SUCCEEDED_FORKS=()
 
 for fork in "${TARGET_FORKS[@]}"; do
-    fork_dir="local/sources/${fork}"
+    fork_dir="$PROJECT_ROOT/local/sources/${fork}"
 
     echo -e "${BLUE}=== ${fork} ===${NC}"