/squad:dashboard: ship the team dashboard in the Claude Code plugin #2460

Part of #2438. Belongs to Phase 5 (#2443). Needs the live-session reader (#2459).

Goal

Ship the team dashboard with the Squad Claude Code plugin, so a user with the plugin can open it with one command.

What needs to be done

• A /squad:dashboard slash command in the plugin. It starts the dashboard server for the current repo on a free local port (127.0.0.1 only), opens the browser, and prints the URL.
• If a dashboard is already running for this repo, reuse it rather than starting another; stop it when asked (for example /squad:dashboard stop), and don't leave orphan processes when the session ends.
• Bundle the server and page in the plugin with no runtime dependencies beyond Node (the prototype already needs none).
• A matching CLI entry, for example squad dashboard, for users of squad init --client claude without the plugin. The epic keeps both install paths working.
• Docs: a short page on what the dashboard shows (Board, Roadmap, Conversation), how to open it, and that it reads Claude Code's local session files and never sends them anywhere.
• Tests: the command starts the server, and the port, reuse and stop handling work; plus a smoke test that the page loads and the SSE endpoint streams events from a fixture session.

Done when

• /squad:dashboard works in an interactive Claude Code session with the plugin installed, and squad dashboard works without it.
• It's documented, and it's covered by the plugin's tests.