feat: structured logging for provider diagnostics + Cursor SDK rules/skills logs

[rfhold]

Summary

The plugin's own diagnostics, and diagnostics from the bundled @cursor/sdk, were being written straight to console.warn/console.error/console.log instead of using opencode's plugin logging API. This surfaced as raw, unstructured lines in the terminal, e.g.:

AgentSkillsCursorRulesService load completed meta={durationMs: 86, ruleCount: 18, skillCount: 18}
LocalCursorRulesService load completed meta={durationMs: 89, ruleCount: 1}

These two (plus a third, CursorPluginsAgentSkillsService load completed) come from @cursor/sdk's bundled local-exec runtime, which writes them straight to console.log — there's no public logger hook in the SDK to redirect them.

What changed

• Plugin's own logging: every console.warn/console.error call in agent-backend.ts, agent-events.ts, and language-model.ts now routes through client.app.log({ body: { service: "opencode-cursor", level, message, extra } }), with a console.* fallback when no client bridge is available (e.g. the provider used standalone/in tests).
• New src/provider/log-bridge.ts: mirrors the existing subagent-bridge.ts globalThis pattern so the provider layer (which has no direct import path to the plugin's opencode client) can log through it. Published by plugin/index.ts on init, cleared on dispose().
• New src/provider/cursor-log-intercept.ts: a narrowly-scoped console.log interceptor used for the in-process transport. It matches only the three known Cursor rules/skills "load completed" messages (ANSI-safe) and re-emits them via pluginLog; every other console.log call passes through untouched.
• Sidecar transport: src/sidecar/agent-host.mjs (the Node child that hosts @cursor/sdk when Bun's http2 client can't be used) installs the same interception in the child process and forwards matches to the parent over the existing JSONL protocol as a new { ev: "log" } message. SidecarClient gained an onLog option to receive these.

Testing

• npm run typecheck — clean
• npm test — 371 passed (29 pre-existing files + 2 new: log-bridge.test.ts, cursor-log-intercept.test.ts; sidecar.test.ts gained a log-forwarding case backed by a new emitRulesLog flag in the fake SDK fixture)
• npm run build — verified the interceptor + log bridge are present in both dist/chunk-*.js (shared by provider/plugin) and dist/sidecar/agent-host.js

Notes

A broader interception of every Cursor SDK console.log call would risk swallowing unrelated output; this only recognizes the three confirmed rules/skills messages. If Cursor exposes a proper logger callback upstream, we can drop the interceptor and pass it through directly.