feat: rewrite main.ts for multi-session tool registration

Replace singleton browser/context with SessionManager. All existing
browser tools now receive a mandatory sessionId parameter injected
transparently via registerBrowserTool() wrapper — original tool
handlers remain unmodified.

- registerBrowserTool(): injects sessionId into Zod schema, resolves
  correct session's McpContext, acquires per-session mutex
- registerSessionTool(): full implementations for create/list/close
- Signal handling with process.once() and 10s shutdown timeout
- Safety check rejects tools that define their own sessionId schema

Author: RobertWsp