hive-matrix-mcp: read the main account's token from the store too

The daemon reads each account's token from `swarm/agents/<agent>/matrix/`
as the agent itself, inside its own container, and falls back to the file
only when the store has none. This is #4519's read, without its `main`
carve-out: the swarm now mints `main` there and no hive writes the file.

The daemon unit gets the agent's store identity, spelled the way
forge-token.nix spells it. A timer re-starts it while it is down: a token
the swarm mints or replaces in the store changes no file, so the path
watcher never fires for it, and a daemon that exited on a replaced token
would otherwise stay down until the container restarts.

hive-matrix-mcp/src/lib.rs

@@ -9,6 +9,7 @@
 
 pub mod accounts;
 pub mod client;
+pub mod credential;
 pub mod handlers;
 pub mod mcp;
 pub mod paths;