Replace vendor/cultcache-py submodule with published PyPI dependency

CultLib's packages/cultcache-py is now published as cultcache-py==0.2.0 on
PyPI (MIT), so the standalone GameCult/cultcache-py fork that Ghostlight
vendored as a submodule is retired. Symbol-for-symbol and on-disk-format
verification against the pinned submodule commit 9b4bbb8 found no
incompatibility: every symbol tools/ghostlight_state_store.py imports
(CultCache, JsonLinesBackingStore, define_document_type) and every method it
calls kept its signature, and JsonLinesBackingStore's on-disk JSONL envelope
fields and sort order are unchanged, confirmed by a real round-trip against
Ghostlight's production state/ghostlight-state.cultcache.jsonl.

tools/ghostlight_state_store.py no longer pushes vendor/cultcache-py/src onto
sys.path; it imports cultcache_py directly. Since Ghostlight's session
bootstrap previously depended on the submodule needing no install step,
tools/requirements.txt now pins cultcache-py==0.2.0 and npm run
state:bootstrap (pip install -r tools/requirements.txt, idempotent) runs as a
pre-hook ahead of state:status and state:prepare-compaction; AGENTS.md's
Session Bootstrap protocol and Useful Commands section call it out explicitly
for the direct ghostlight_state.py CLI invocations that don't go through npm.

Verified for real: `python tools/ghostlight_state.py status` and `python
tools/ghostlight_prepare_compaction.py`, run from a venv with cultcache-py
installed from PyPI, against Ghostlight's actual production state file (22 ok
findings, 2 pre-existing unrelated warnings, exit 0 both times).

Idunn's deployment/idunn/recipe.toml never declared vendor/cultcache-py in
required_gitlinks (only vendor/eve) and its build/test/service steps never
touch Python state tooling, so no recipe change is needed for admission.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "vendor/cultcache-py"]
-	path = vendor/cultcache-py
-	url = https://github.com/GameCult/cultcache-py.git
 [submodule "vendor/eve"]
 	path = vendor/eve
 	url = https://github.com/GameCult/Eve.git