refactor(plugin): return tool output directly

[opencode-agent]

Summary

• make typed V2 tool executors return { structured, content } directly
• remove toModelOutput, structured, and toStructuredOutput from the typed tool config
• migrate built-in tools, plugin examples, tests, and the V2 tool spec to the unified result shape

Testing

• bun typecheck in packages/plugin
• bun typecheck in packages/core
• bun typecheck in packages/sdk-next
• bun test --only-failures test/session-runner-tool-registry.test.ts test/plugin.test.ts test/tool-*.test.ts test/session-runner.test.ts in packages/core (222 pass)
• bun test --timeout 5000 test/embedded.test.ts in packages/sdk-next (4 pass)

The repository-wide pre-push typecheck was attempted but the host hit its OS thread limit in @opencode-ai/console-app; all affected packages typecheck cleanly.