feat(memory_store): auto-fallback to SQLite when DATABASE_URL absent

Completes the Glama-inspectability work paired with commit A
(plugin.json env schema). When Cortex launches with no DATABASE_URL
(sandbox inspection, CI smoke, first-glance experimenter), the
factory now logs a warning and drops to the existing SQLite backend
rather than raising RuntimeError.

Behaviour matrix:
  DATABASE_URL set + reachable         → PostgreSQL (unchanged)
  DATABASE_URL set + unreachable       → raise (unchanged — signals
                                          real config error)
  DATABASE_URL unset / empty           → SQLite with loud warning
                                          (was: raise)
  CORTEX_ALLOW_SQLITE_FALLBACK=1       → SQLite even if PG fails

Rationale: real production users who ran /cortex-setup-project have
DATABASE_URL set to a reachable Postgres, so their path is
unchanged. Misconfigured Postgres still raises loudly. The only
path that flips is "no DATABASE_URL at all," which today leaves
users with an unbootable server — now it boots against SQLite so
tools are discoverable and the install can be tried before wiring
Postgres.

Unlocks Glama's sandbox inspector → expected Quality grade lift
C → B/A once their next evaluation runs.

Author: cdeust

mcp_server/infrastructure/memory_store.py

@@ -70,13 +70,36 @@ def __new__(
                 store, err = None, "DATABASE_URL not set"
             if store is not None:
                 return store
+            # Inspection-mode fallback — Glama's sandbox, CI smoke
+            # tests, and first-glance experimenters launch Cortex with
+            # no DATABASE_URL. Rather than hard-fail and leave them
+            # unable to even see the tool surface, drop to SQLite with
+            # a loud warning. Real production users who have
+            # configured Postgres will see the PG connect succeed;
+            # only unset/unreachable installs trip this path.
+            allow_fallback = (
+                not url
+                or os.environ.get("CORTEX_ALLOW_SQLITE_FALLBACK", "").lower()
+                in ("1", "true", "yes")
+            )
+            if allow_fallback:
+                logger.warning(
+                    "PostgreSQL unavailable (%s); falling back to SQLite. "
+                    "This is expected for inspection/sandbox launches; "
+                    "production installs should set DATABASE_URL.",
+                    err,
+                )
+                return _make_sqlite(
+                    db_path or settings.SQLITE_FALLBACK_PATH, embedding_dim
+                )
             raise RuntimeError(
                 f"PostgreSQL connection failed (url={url or '<unset>'}): {err}\n"
                 "Cortex requires PostgreSQL in CLI mode.\n"
                 "Run: bash setup.sh to configure PostgreSQL.\n"
                 "If DATABASE_URL is set, verify it points to a reachable Postgres instance "
                 "(host/port/credentials/database exists).\n"
-                "Or set CORTEX_RUNTIME=cowork to allow SQLite fallback."
+                "Or set CORTEX_RUNTIME=cowork (or CORTEX_ALLOW_SQLITE_FALLBACK=1) "
+                "to allow SQLite fallback."
             )
 
         # "auto" in cowork mode: try PG, fall back to SQLite