docs(styleguide): add chroma_cache.md — chroma DB path and cleanup pattern

Lesson 5 from the 4-day test-hell saga. The chroma cache lives at
tests/artifacts/.slop_cache/chroma_<collection>/, NOT at the per-run
live_gui_workspace_<timestamp>/ subdir. The trailing-slash bug in
Path(active_project_path).parent places the cache one level higher
than expected.

RAG tests must pre-clean the cache to avoid persistent state from
prior batched runs. Documents the cleanup pattern (shutil.rmtree with
ignore_errors=True), the auto-recovery mechanism (_validate_collection_dim),
and 3 anti-patterns (assuming per-run, not cleaning, asserting on
first chunk in batched context).

conductor/code_styleguides/chroma_cache.md

@@ -0,0 +1,90 @@
+# Chroma Cache Path Styleguide
+
+## The Rule
+
+The ChromaDB persistent vector cache lives at:
+
+```
+<project_root>/tests/artifacts/.slop_cache/chroma_<collection_name>/
+```
+
+**NOT** at the per-run `tests/artifacts/live_gui_workspace_<timestamp>/` subdir.
+
+Tests that interact with RAG **MUST** pre-clean the cache to avoid persistent state from prior tests in the batched run.
+
+## Why This Rule Exists
+
+The chroma cache path is auto-derived from `RAGEngine._init_vector_store()` (`src/rag_engine.py:108-125`):
+
+```python
+db_path = os.path.abspath(os.path.join(
+    self.base_dir, ".slop_cache", f"chroma_{vs_config.collection_name}"
+))
+```
+
+`self.base_dir` is computed as `Path(active_project_path).parent`. **The trailing-slash bug**: when the test config produces a project path ending in `/` (e.g., from `os.path.join` with a trailing `/`), `Path(p).parent` returns the directory ONE LEVEL HIGHER than expected. So the chroma cache lands at `tests/artifacts/.slop_cache/` (the parent of the per-run `live_gui_workspace_<timestamp>/` subdir) instead of inside the per-run subdir.
+
+This was the dominant cause of `tier-3-live_gui` failures in the 2026-06-08 to 2026-06-10 window. A prior batched run with a different embedding provider (e.g., Gemini 3072-dim vs local 384-dim) leaves a corrupt collection on disk. The next test's `search()` raises `chromadb.errors.InvalidDimensionError: Collection expecting embedding with dimension of X, got Y`, the AI request never reaches `'done'` status, and the live_gui test polls timeout at 50×0.5s = 25s.
+
+## The Pre-Cleanup Pattern
+
+RAG tests should wipe the chroma cache BEFORE pushing RAG config. The pattern is in `tests/test_rag_phase4_final_verify.py`:
+
+```python
+from pathlib import Path
+import shutil
+
+def test_phase4_final_verify(live_gui):
+    # Wipe any stale chroma from prior batched runs
+    cache = Path("tests/artifacts/.slop_cache/chroma_test_final_verify")
+    if cache.exists():
+        shutil.rmtree(cache, ignore_errors=True)
+    # ... rest of test
+```
+
+`ignore_errors=True` is required because:
+- On Windows, the chroma client may still hold file handles; `rmtree` may fail with `WinError 32` (sharing violation).
+- If a parallel xdist worker is mid-write, the rmtree can race; `ignore_errors` lets the next worker's write retry.
+
+The `_validate_collection_dim()` mechanism in `RAGEngine` (`src/rag_engine.py:127-213`) also auto-recovers by wiping the dim-mismatched collection (see [docs/guide_rag.md](../docs/guide_rag.md#dimension-mismatch-protection)). But pre-cleaning is faster and avoids the stderr warning.
+
+## Anti-Patterns
+
+❌ **Assuming the cache is per-run:**
+```python
+def test_rag(live_gui, live_gui_workspace):
+    # WRONG: live_gui_workspace is a per-run subdir, but the chroma
+    # cache is at tests/artifacts/.slop_cache/, NOT under live_gui_workspace
+    cache = live_gui_workspace / ".slop_cache" / "chroma_test"
+    if cache.exists():
+        shutil.rmtree(cache)  # Doesn't find the actual cache
+```
+
+❌ **Not pre-cleaning at all:**
+```python
+def test_rag(live_gui):
+    # WRONG: no pre-cleanup. If a prior batched run with a different
+    # embedding provider is on disk, this test will hit dim-mismatch
+    client = ApiHookClient()
+    client.push_event("set_value", {"field": "rag_enabled", "value": True})
+    # ... eventually hangs polling for 'done' status
+```
+
+❌ **Asserting on the FIRST retrieved chunk:**
+```python
+assert "Manual Slop RAG is great" in entry.get("content")
+# WRONG: in batched context, the chroma ordering may rank a .py
+# file first instead of the .txt file. Either file's content
+# proves RAG worked; the assertion must accept either.
+```
+
+## When in Doubt
+
+If a RAG test is flaky in batched runs but passes in isolation, the chroma cache is the #1 suspect. The test's actual chroma path is `Path("tests/artifacts/.slop_cache") / f"chroma_{collection_name}"`. Wipe it before the test starts.
+
+## Related
+
+- [docs/guide_testing.md §Chroma Cache Path and Cross-Test Pollution](../docs/guide_testing.md) — broader context in the testing guide
+- [docs/guide_rag.md §Dimension Mismatch Protection](../docs/guide_rag.md) — the auto-recovery mechanism
+- [conductor/code_styleguides/workspace_paths.md](./workspace_paths.md) — sibling styleguide for test workspace paths
+- [docs/reports/test_infrastructure_hardening_batch_green_20260610.md](../docs/reports/test_infrastructure_hardening_batch_green_20260610.md) — the 6-lesson summary this styleguide is sourced from