Feat v0.8.0 files sandbox module

Bridge ObjectStore (PVC/S3) into the Starlark sandbox with extension-scoped
key namespacing. New files module with put/get/meta/list/delete/exists
builtins gated by files.read and files.write permissions.

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

docs/STARLARK-REFERENCE.md

@@ -275,6 +275,48 @@ results, errors = batch.exec([
 
 ---
 
+### files
+
+**Permissions:** `files.read`, `files.write`
+
+Store and retrieve files via the kernel ObjectStore (PVC or S3).
+All keys are scoped to `ext/{packageID}/` — extensions cannot access
+each other's files.
+
+```python
+# Store a file with optional metadata
+files.put("reports/q1.pdf", pdf_bytes,
+          content_type="application/pdf",
+          metadata={"quarter": "Q1", "year": 2026})
+
+# Read a file
+result = files.get("reports/q1.pdf")
+# result = {"content": b"...", "content_type": "application/pdf",
+#           "size": 12345, "metadata": {"quarter": "Q1", "year": 2026}}
+
+# Metadata only (no content transfer)
+meta = files.meta("reports/q1.pdf")
+
+# List files by prefix
+entries = files.list(prefix="reports/", limit=50)
+# entries = [{"name": "reports/q1.pdf", "size": 12345, "content_type": "..."}]
+
+# Check existence
+if files.exists("reports/q1.pdf"):
+    files.delete("reports/q1.pdf")
+
+# Bulk delete
+files.delete_prefix("temp/")
+```
+
+**Notes:**
+- `content` accepts string or bytes; `get()` returns bytes.
+- Maximum file size: 50 MB (configurable via `EXT_FILES_MAX_SIZE`).
+- Metadata is stored as a companion JSON object, not in the file itself.
+- `files.list()` automatically filters out internal metadata companions.
+
+---
+
 ## Example: automated stage hook
 
 A simple hook that reads a setting, queries data, and advances: