Studio Code: Add Opus 4.8, remove Opus 4.6

[youknowriad]

Related issues

• Related to #

How AI was used in this PR

Implemented with Claude. AI updated the agent model registry and the session model-resolution logic, then verified there were no other references to the removed model id across the codebase. Reviewers should pay particular attention to the resume/auto-switch behavior in resolveSessionModel and the CLI resume path.

Proposed Changes

• Adds Opus 4.8 to the Studio Code agent's selectable models and removes Opus 4.6, keeping the model picker aligned with the models we actually support.
• Sessions that were last using a model we no longer offer (e.g. Opus 4.6) now auto-switch to the default model when resumed, instead of pinning a model that's gone. The most recently recorded model wins, and an unknown/removed model falls back to the default. Because the fallback points at the default model rather than a hardcoded id, changing the default later automatically updates where these sessions land.
• Both the desktop UI and the CLI resume path share the same resolution logic, so behavior is consistent across surfaces.

Testing Instructions

• Open a chat in Studio Code and confirm the model picker lists Sonnet 4.6, Opus 4.7, Opus 4.8, and GPT 5.5, with no Opus 4.6 entry.
• Resume an existing session that was last on Opus 4.6 and confirm it comes back on the default model (Sonnet 4.6) rather than erroring or showing a missing model.
• Resume a session on a still-supported model (e.g. Opus 4.7) and confirm it resumes on that same model.
• Start a brand-new session and confirm it uses the default model.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[youknowriad]

I'm not sure if Opus produces better results than Sonnet. I prefer Sonnet but still this adds the latest Opus at least.

[wpmobilebot]

📊 Performance Test Results

Comparing fd97618 vs trunk

app-size

Metric	trunk	fd97618	Diff	Change
App Size (Mac)	1335.91 MB	1335.91 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	fd97618	Diff	Change
load	1738 ms	1744 ms	+6 ms	⚪ 0.0%

site-startup

Metric	trunk	fd97618	Diff	Change
siteCreation	9076 ms	9075 ms	1 ms	⚪ 0.0%
siteStartup	4411 ms	4875 ms	+464 ms	🔴 10.5%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)