chore(plugin): declare CORTEX_RUNTIME in mcp env schema

Glama's inspector reads plugin.json's env block to know what
variables the server accepts. Declaring CORTEX_RUNTIME (empty
default) surfaces it in Glama's Try-in-Browser UI so a user can
select cowork mode and boot Cortex without a Postgres instance.

No runtime behaviour change on its own — the var is already honoured
by memory_config._detect_runtime(). This only makes it discoverable
to Glama's sandbox UI. Paired with the memory_store auto-fallback
(commit B following) for the case where the sandbox sets no env at
all.

Author: cdeust

.claude-plugin/plugin.json

@@ -15,7 +15,8 @@
       "command": "uvx",
       "args": ["--python", "3.13", "--from", "neuro-cortex-memory[postgresql]", "neuro-cortex-memory"],
       "env": {
-        "DATABASE_URL": "postgresql://localhost:5432/cortex"
+        "DATABASE_URL": "postgresql://localhost:5432/cortex",
+        "CORTEX_RUNTIME": ""
       }
     }
   },