fix(mcp): select the inbox by direction, not by a LIKE on the agent's own name

murmur_inbox called searchMessages(agentConfig.agentId) — a LIKE over
text/sender/conversation_id — and then filtered the hits by direction. A reply
that did not happen to spell out the receiving agent's name matched nothing, so
the tool answered count:0 while the row sat in local_messages and the sender saw
the delivery acked. Reported by @lichtpfad from a Mac/Windows cross-host run:
claude 4 inbound -> 0, codex 3 -> 1 (only the one containing the word "codex"),
automata 2 -> 0.

The store is per-agent, so direction is the whole filter. Adds
SQLiteMessageStore.listInbound(limit) and uses it. searchMessages keeps its
existing meaning for the search_messages tool.

Closes #114