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.

node/tests/cli-hook.test.ts

@@ -92,7 +92,8 @@ describe("`session-announce` CLI behavior", () => {
     });
     expect(r.status).toBe(0);
     expect(r.stdout).toContain("`claude-abc12345`");
-    expect(r.stdout).toContain("claude-mailbox send");
+    expect(r.stdout).toContain("mcp__mailbox__send");
+    expect(r.stdout).toContain(`from="claude-abc12345"`);
   });
 
   it("uses base prefix when set", () => {