test: preserve community regressions and Goal Mode checks

[ByBrawe]

Why

A quick audit of community-reported issues found two closed bug reports:

• Plugin fails to load silently when used as .ts file due to const reassignment #2: the installer copies src/index.js to opencode-loop.ts, but CI only imported the .js source with Bun. The original failure mode therefore was not directly protected by a regression gate.
• Running background tasks should be considered busy, not idle. #4: long-running tools/background child sessions were already fixed in v0.5.16 and still have explicit smoke-test coverage today.

Goal Mode already has broad lifecycle coverage, but the headline --check ... --complete-when-checks-pass workflow did not have an explicit smoke regression.

Changes

• add a Bun CI regression that copies the runtime source to a .ts file exactly like the installer does and imports that TypeScript-shaped local plugin on Ubuntu, Windows, OpenCode peer 1.4.0, and latest;
• preserve the existing Running background tasks should be considered busy, not idle. #4 active-tool/background-child regressions unchanged;
• add an explicit Goal Mode regression proving successful configured checks can auto-complete, pause, and disable the goal through --complete-when-checks-pass;
• no runtime behavior or package version change.

Goal Mode README promotion is intentionally handled as a separate self-cleaning docs change after this regression PR is green, so temporary workflow machinery does not remain in the final test diff.

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