Add a way to list currently running Gemini CLI sessions with their status

Feature request: Anthropic's Claude Code CLI has a ⁠claude agents --json⁠ command that lists every currently-running interactive session on the machine, each with a live status (idle/busy/waiting for input). This enables building an external multi-session dashboard.
Gemini CLI's ⁠--list-sessions⁠/⁠--resume⁠/⁠/resume⁠ are all about saved conversation history for resuming a specific past session - there's no way to discover other currently-running ⁠gemini⁠ processes or their live state. ⁠--acp⁠ mode is a JSON-RPC server, but it requires the caller to spawn and own the process itself, so it can't be used to observe independently-started terminal sessions either.
Ask: a lightweight way to enumerate currently-active Gemini CLI sessions (id, cwd, busy/idle status) from outside the process - even a simple discovery file (PID + status, similar to how the IDE companion writes its own discovery file) would enable external tooling to build a live status dashboard across parallel sessions.

Start by reviewing the existing --list-sessions, --resume, and /resume entry points, then compare how the IDE companion writes its discovery file. Define how independently started Gemini processes are discovered and how id, cwd, and busy/idle status remain accurate; done means an external dashboard can enumerate active sessions without spawning them.

Written by the indexing model from the issue text.