fix: thread-local CRDT and content store for test isolation

Tests shared a global CRDT singleton and content store HashMap, causing
flaky failures when parallel tests wrote items that polluted each
other's assertions. 3-5 random test failures per run.

Both CRDT_STATE and CONTENT_STORE now use thread_local! in test mode
so each test thread gets its own isolated instance. Production code
is unchanged — it still uses the global OnceLock singletons.

Also fixed 3 tests (create_story_writes_correct_content,
next_item_number_increments_from_existing_bugs,
next_item_number_scans_archived_too) that relied on leaked state
from other tests — they now write to the content store explicitly.

Result: 1902 passed, 0 failed across 5 consecutive runs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/http/workflow/story_ops.rs

@@ -315,10 +315,13 @@ mod tests {
 
     #[test]
     fn create_story_writes_correct_content() {
+        crate::db::ensure_content_store();
         let tmp = tempfile::tempdir().unwrap();
         let backlog = tmp.path().join(".huskies/work/1_backlog");
         fs::create_dir_all(&backlog).unwrap();
         fs::write(backlog.join("36_story_existing.md"), "").unwrap();
+        // Also write to content store so next_item_number sees it.
+        crate::db::write_item_with_content("36_story_existing", "1_backlog", "---\nname: Existing\n---\n");
 
         let number = super::super::next_item_number(tmp.path()).unwrap();
         // The number must be >= 37 (at least higher than the existing "36_story_existing.md"),