feat: flesh out OpenClaw websocket mock across the operator API

[outofcoffee]

Expand the OpenClaw Gateway websocket mock from a handful of methods to the full operator-facing API, split by domain.

Summary

• Cover ~60 operator methods across agents, sessions, chat, models, cron, commands/tools/skills, channels, tasks/artifacts and system/audit/config, with canonical req/res/event frames that echo the request id.
• Model one self-consistent world: a single main agent and main session, three models, one cron job, one task, one artifact and one connected channel account; reads return this snapshot, writes are acknowledged, and unmocked methods return a method_not_found error frame.
• Split the config into per-domain *-config.yaml files (sessions, chat, agents, cron, tools, channels, tasks, system) plus a connection core, with matching payload files.
• Fix the streamed chat delta to use the canonical deltaText field and expand hello-ok to advertise the full method set.
• Rewrite the README to document every domain with paste-able frames and the split-file layout.

Implementation details

Method payload shapes are taken from the OpenClaw gateway protocol schemas rather than invented. A few results are not pinned by the wire schema (e.g. sessions.list, sessions.usage, cron.status); those use representative shapes consistent with the reference client, noted in the README.

Splitting relies on the engine coalescing multiple plugin: websocket configs onto one connection (imposter-project/imposter-go#95); until that lands, an engine build including it is required.