Tell the Chat agent which sign-ins it holds, and read the room thread first #833

An operator's Chat agent does not know which sign-ins its sandbox holds, and in a Slack room it can answer without reading the thread.

Example: in a Slack thread, people asked their agents to read a private repository with gh. The operator's sandbox held their GitHub sign-in (#769), so gh worked as them. The operator tagged their agent: "you should have gh cli authenticated with my account". The agent made no tool call. It did not run gh auth status and did not call chat_read_thread. It looked at its working directory, saw no git repository, and answered that there was nothing to authenticate against.

Two gaps:

• The agent does not know what it can reach. Nothing in the Chat prompt says which services the sandbox holds a sign-in for, so the agent infers from its empty working directory that it has no GitHub access.
• Reading the thread is optional. For Slack, the prompt says "In a room, other people also write in the thread: call chat_read_thread to read it." The agent answered a one-line message from a long thread without its context.

Expected

• The Chat prompt names the sign-ins the sandbox holds, for each service with a host: for example, "gh is signed in as , the person's GitHub account". Without a sign-in, it says how to connect one (Chat → Channels).
• In a room, the agent reads the thread with chat_read_thread before it answers a message whose subject is not in the message itself.