Add `robot.*` RPC methods and stream topics to `ralph-api`

[moltar]

With the introduction of WebRobotService (#242), the orchestration loop can communicate via JSON files in .ralph/api/. However, there's currently no way for external systems to interact with these files through ralph-api — no RPC methods to read pending questions, submit responses, or send guidance over HTTP.

Problem

For a remote control plane (or any HTTP client) to participate in Ralph's human-in-the-loop flow, ralph-api needs endpoints that bridge the HTTP world to the file-based WebRobotService. Without these, the only way to interact is by directly writing files — which doesn't work for remote systems.

Proposed RPC methods

New robot.* domain in ralph-api:

Method	Purpose
robot.question	Returns the pending question from .ralph/api/robot-question.json, or null if none
robot.respond	Writes the response to .ralph/api/robot-response.json so the running loop picks it up. Publishes robot.response.received to the event stream
robot.guidance	Appends a human.guidance event to the active events JSONL file (found via .ralph/current-events). Publishes robot.guidance.sent to the stream
robot.checkin	Returns the latest checkin data from .ralph/api/robot-checkin.json (iteration, elapsed, hat)

Proposed stream topics

Topic	When	Payload
robot.question.asked	WebRobotService writes a question file	{ question_id, payload, hat, iteration }
robot.response.received	robot.respond is called	{ question_id, response }
robot.guidance.sent	robot.guidance is called	{ text }
robot.checkin.updated	WebRobotService writes a checkin file	{ iteration, elapsed_secs, hat }

The WebRobotService can publish robot.question.asked and robot.checkin.updated via the existing _internal.publish RPC method (already available for orchestration loop → API communication).

Key files

• crates/ralph-api/src/runtime/dispatch.rs — method routing (add robot.* family)
• crates/ralph-api/src/stream_domain/mod.rs — stream topic definitions
• crates/ralph-api/src/robot_domain.rs — new domain module

Depends on

• WebRobotService: file-based RobotService for API-driven human interaction #242 (WebRobotService)