MCP: retry send_input without typing the text twice #385

As an agent coordinating several sessions through agent-guild mcp --allow-input, I want to retry send_input safely after an unclear failure, so that text I meant to type once is never typed twice.

Current behavior

• send_input returns sent: true once the text has been sent to the manager. Nothing identifies a particular send.
• If a call fails without a clear result (for example "The terminal closed before the text was sent", or the coordinator's own client times out), the coordinator can't tell whether the text reached the terminal.
• A retry may type the text again: a second prompt to the agent, or a second answer such as y.

Acceptance criteria

• A coordinator can retry a send_input call and be sure the text is typed at most once.
• A coordinator can tell whether an earlier call's text reached the terminal.
• send_input without this behaves as it does today.
• docs/mcp.md (and docs/api.md, if the local API changes) describe the behavior, and tests cover it.

To decide

• How long the manager remembers a send it has already applied.

Starting points

• send_input in src/mcp/server.mjs