Support more agents (gemini-cli, aider, opencode)

[huiyu]

Add agent adapters under `lib/agents/` for more terminal coding agents as their hook support lands:

• `gemini-cli`
• `aider`
• `opencode`

Each adapter mirrors the existing `lib/agents/claude.sh` / `lib/agents/codex.sh`: map the agent's lifecycle events to agentdeck states (working / waiting / idle / gone) and expose the right field selectors for session id / cwd / transcript. Blocked per-agent on the upstream tool exposing usable hooks or a notify mechanism.

Refs: README Roadmap (Later).