refactor: split http/workflow/story_ops.rs (1256) into create + criterion + update

The 1256-line story_ops.rs is split:

- create.rs: create_story_file + tests (~232 lines)
- criterion.rs: check/add/remove/edit_criterion_in_file + tests (~525 lines)
- update.rs: update_story_in_file + yaml helpers + tests (~640 lines)
- mod.rs: re-exports (~12 lines)

Workflow helpers (read_story_content, write_story_content, slugify_name, etc.)
bumped from pub(super) to pub(crate) since they're now consumed across nested
sub-modules and from http/mcp/story_tools/.

Tests stay co-located. All 2636 tests pass; clippy clean.

server/src/http/workflow/story_ops/mod.rs

@@ -0,0 +1,12 @@
+//! Story operations — creates, updates, and manages acceptance criteria in story files.
+
+mod create;
+mod criterion;
+mod update;
+
+pub use create::create_story_file;
+pub use criterion::{
+    add_criterion_to_file, check_criterion_in_file, edit_criterion_in_file,
+    remove_criterion_from_file,
+};
+pub use update::update_story_in_file;