feat(mcp): identity-via-arg + plugin ships .mcp.json

MCP tools (send/check_inbox/peek_inbox/list_mailboxes) now accept the
caller's mailbox name as an explicit argument (from/name), falling back
to the X-Mailbox header for legacy single-session HTTP setups. This
unblocks multi-session coordination through a shared .mcp.json — each
Claude session passes its own session-derived name on every call,
instead of relying on a single transport header that all sessions
would share.

The plugin now ships .mcp.json (no header), and the SessionStart
announcement spells out the exact args to pass to each mcp__mailbox__*
tool so Claude wires it up automatically.

plugin/.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "mailbox": {
+      "type": "http",
+      "url": "http://127.0.0.1:47822/mcp"
+    }
+  }
+}