refactor: use project-named state file and store prompt/params directly

- State filename now derives from config: cards.bulkgen.yaml produces
  .cards.bulkgen-state.yaml instead of .bulkgen.state.yaml
- Store resolved prompt text and extra params directly in state file
  instead of hashing them, making state files human-readable
- Only file input contents remain hashed (SHA-256)
- Thread project_name through builder and CLI
- Remove hash_string() and _extra_hash() helpers
- Update .gitignore pattern to .*.bulkgen-state.yaml

bulkgen/builder.py

@@ -147,6 +147,7 @@ async def _build_single_target(
 async def run_build(
     config: ProjectConfig,
     project_dir: Path,
+    project_name: str,
     target: str | None = None,
     on_progress: ProgressCallback = _noop_callback,
 ) -> BuildResult:
@@ -171,7 +172,7 @@ async def run_build(
             raise ValueError(msg)
         graph = get_subgraph_for_target(graph, target)
 
-    state = load_state(project_dir)
+    state = load_state(project_dir, project_name)
     generations = get_build_order(graph)
     target_names = set(config.targets)
 
@@ -209,7 +210,7 @@ async def run_build(
         )
 
         _process_outcomes(outcomes, config, project_dir, state, result, on_progress)
-        save_state(state, project_dir)
+        save_state(state, project_dir, project_name)
 
     return result