fix(agents): recognize GPT-6 model variants

[Filip-Stastny-Philips]

Summary

Add GPT-6 model recognition for CodeMie OpenCode and Pi integrations.

Changes

• Route GPT-6.1, GPT-6 Sol, and other GPT-6 variants through the OpenAI Responses API.
• Assign the 1,050,000 context and 128,000 output token limits.
• Keep vendor-prefixed identifiers such as openai.gpt-6.1 supported by the existing regex approach.

Testing

• npm run lint
• npm run typecheck
• Related unit tests passed through the pre-commit hook.
• Local secrets scan was skipped because no container engine is installed; CI should run it.

[Siarhei-Yarkavy]

looks cover the same as https://github.com/codemie-ai/codemie-code/pull/582/changes

[Filip-Stastny-Philips]

looks cover the same as https://github.com/codemie-ai/codemie-code/pull/582/changes

@Siarhei-Yarkavy 80% same, but Pi is different: PR #582 uses incorrect name filter, asserting "name starts with gpt-6...", but we need the same logic as in opencode filter "model contains gpt-6..."

If #582 is merged, I can update this PR to only change that issue. Or if you fix the filter in #582 (I have no permissions), we can close this one. But pls let's not delay #582 or this PR too much, GPT-6 models are currently unusable in codemie-code

[Siarhei-Yarkavy]

@Filip-Stastny-Philips I updated the PR, it should be fine now. bb3fe16

[Filip-Stastny-Philips]

All fixes ported to #582 --> closing this one