fix(opencode): warn on unknown config fields

[opencode-agent]

Summary

• detect unrecognized top-level fields before V1 decodes a config
• emit one structured warning per config source with the ignored field names
• continue loading the config and discarding those fields

This follows the forward-compatible parsing added in #41312 while keeping configuration typos visible in logs. For example, V1 can start with V2's plugins field and logs that it ignored the field.

Testing

• bun test test/config/config.test.ts (96 passed)
• bun x prettier --check src/config/parse.ts src/config/config.ts test/config/config.test.ts
• bun typecheck attempted but did not complete within the 3-minute local timeout

Requested by: @kitlangton (Kit via Slack)

[github-actions]

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