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/src/cli.ts

@@ -201,10 +201,17 @@ program
     if (!sid) return;
     const base = (process.env["CLAUDE_MAILBOX_NAME"] ?? "").trim() || null;
     const name = deriveSessionName(sid, base);
-    process.stdout.write(
-      `Claude-Mailbox: this session is mailbox \`${name}\`. ` +
-        `Peers can send to it with: claude-mailbox send --from <peer> --to ${name} --body "..."\n`,
-    );
+    const lines = [
+      `Claude-Mailbox: your mailbox name this session is \`${name}\`.`,
+      `When using mcp__mailbox__* tools, ALWAYS pass this name explicitly:`,
+      `  - mcp__mailbox__send: from="${name}"`,
+      `  - mcp__mailbox__check_inbox: name="${name}"`,
+      `  - mcp__mailbox__peek_inbox: name="${name}"`,
+      `  - mcp__mailbox__list_mailboxes: name="${name}"`,
+      `Peers reach you with: mcp__mailbox__send(from="<their-name>", to="${name}", body="...")`,
+      ``,
+    ];
+    process.stdout.write(lines.join("\n"));
   });
 
 program