[FEATURE]: Add generation completion sentinels

Feature hasn't been suggested before.

• I checked that there is no active issue. This replaces #25921, which was closed as stale.

Describe the enhancement you want to request

I previously opened #25921 and #25924 for this. The PR passed its checks, and I have kept the branch rebased against dev since then. Automated cleanup closed the PR before a maintainer reviewed it, and the issue was later closed for inactivity.

Current dev still has the same problem. script/generate.ts and packages/sdk/js/script/build.ts can produce a lot of output, but neither prints a clear final success line. If an agent or tool truncates the output, I cannot tell whether generation finished.

The change would stay small:

await $`bun ./packages/sdk/js/script/build.ts`;
console.log("opencode generate: sdk build complete");

await $`bun dev generate > ../sdk/openapi.json`.cwd("packages/opencode");
console.log("opencode generate: openapi export complete");

await $`./script/format.ts`;
console.log("opencode generate: format complete");
console.log("opencode generate complete");

• add logs only after successful steps
• add opencode sdk js build complete to the SDK script because it is also run directly
• leave packages/opencode/src/cli/cmd/generate.ts unchanged because its stdout is redirected to openapi.json

I still think this is worth implementing. I have been keeping branch linked in #25924 continuously maintained and updated against dev, so you can check that out for my reference implementation.

Process: human-directed and reviewed, with agentic assistance from openai/gpt-5.6-sol/high.