Add GLM 5.3 Flash model (Automattician-only)

[youknowriad]

Related issues

How AI was used in this PR

This PR was authored by Claude Code (model registry edit, verification, and PR creation), with the model choice and rollout constraints specified by @youknowriad.

Proposed Changes

• Adds GLM 5.3 Flash (zai-org/GLM-5.3-Flash) to the Studio Code model picker on the hosted family, text-only like the other GLM entries.
• It is Automattician-only for now (requiresAutomattician), so it stays hidden from non-a11n pickers while it is evaluated.
• No runtime changes: providers route by family, so the registry entry is the whole client-side change.

Note for reviewers: picker visibility is client-side only — serving this model also requires the wpcom-side allowlist to include the new id before it works end to end. Until that ships, selecting it returns a 403.

Testing Instructions

• npm test -- apps/cli/ai/tests/pi-runtime.test.ts
• In the app (as an Automattician), open the Studio Code model picker and confirm "GLM 5.3 Flash" is listed; as a non-Automattician account it should not appear.
• End-to-end turns against the model depend on the server-side allowlist update.

Pre-merge Checklist

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

🤖 Generated with Claude Code

[youknowriad]

Actually I'm not interested in Luna anymore. I'm going to simplify this PR

[wpmobilebot]

📊 Performance Test Results

Comparing 8c18d60 vs trunk

app-size

Metric	trunk	8c18d60	Diff	Change
App Size (Mac)	1422.13 MB	1422.13 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	8c18d60	Diff	Change
load	1190 ms	1222 ms	+32 ms	⚪ 0.0%

site-startup

Metric	trunk	8c18d60	Diff	Change
siteCreation	7523 ms	7516 ms	7 ms	⚪ 0.0%
siteStartup	3370 ms	3382 ms	+12 ms	⚪ 0.0%

---

Results are median values from multiple test runs.

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