fix(core): coordinate websearch consent through service

[opencode-agent]

Summary

• add a global WebSearchPreference service that owns the device-wide provider preference and its semaphore
• have Location-scoped websearch instances share that global service instead of reading KV directly
• serialize consent through the service so concurrent searches across Locations still produce one form without module-level state
• retry each search after the shared selection and cover five concurrent calls with a regression test

This is the service-scoped alternative to #40869.

Checks

• bun test test/tool-websearch.test.ts --only-failures (from packages/core)
• GOMAXPROCS=2 bun typecheck (from packages/core)
• bun run lint packages/core/src/websearch-preference.ts packages/core/src/tool/plugin/websearch.ts packages/core/src/plugin/internal.ts packages/core/test/tool-websearch.test.ts
• Prettier check on changed implementation/test files
• git diff --check

Requested by: @jlongster (James Long via Slack)

[github-actions]

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