fix(agents): recover Copilot and OpenCode JSON output

[Arcadia822]

Summary

• switch Copilot final-message parsing from raw JSON.parse to the shared parseAgentJson(...) recovery path
• switch OpenCode final_answer fallback parsing to the same shared recovery path
• add Copilot/OpenCode coverage for prose-prefixed JSON and prose-prefixed fenced JSON

Scope

This intentionally only fixes Copilot and OpenCode, matching the owner suggestion on #147. I did not move parseAgentJson(...) outside adapter implementations in this PR. Long-term, that could make sense as a shared post-adapter normalization layer, but it would be a broader abstraction change than this fix needs.

Local validation

• npx vitest run src/core/agents/copilot.test.ts src/core/agents/opencode.test.ts
• I also used the parseAgentJson(...)-based local build with GLM 5.1 and deep-v4-flash, and both worked as expected.

Addresses #147

Pipeline

Updates from git push no-mistakes

Local no-mistakes gate ran review, test, document, and lint for this branch. The final upstream push step was blocked by fork permissions, so the branch remains published from Arcadia822/gnhf.

[kunchenguid]

Thanks - scoped exactly as discussed in #147, and the helpers track the rovodev shape closely. The fork-blocked no-mistakes check is understood; merging.