[Docs] Document GLM 5.3 provider access

[zoomote]

​Opened on behalf of @taltas. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Added an OpenCode Go provider guide covering GLM-5.3 capabilities, pricing, and setup.
• Updated the Z AI provider guide to document GLM-5.3 availability through International Coding and China Coding Plan entrypoints, plus the separate standard API endpoints where 5.3 is not yet available.
• Documented the provisional GLM-5.2 pricing assumptions used for international and China mainland GLM-5.3 cost estimates.

Why this change was made

GLM-5.3 launched through Z AI's Coding Plan before the standard API and official token pricing. The docs now explain that phased rollout and both supported Zoo Code routes without presenting the standard pay-as-you-go API as available prematurely.

Impact

Users can configure either native Z AI Coding Plan access or OpenCode Go for GLM-5.3 and understand the current limits, always-on reasoning behavior, provisional pricing, and endpoint requirements.

Screenshots

Related PRs

• Zoo Code model support