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.
