docs(providers): add GLM (Z.ai) provider guide

[Rubyglask]

Summary

Adds a provider guide for using GLM (Z.ai) models with CAI, matching the existing docs/providers/ style (Ollama).

GLM speaks an OpenAI-compatible API, so CAI reaches it through two tool-agnostic paths:

• OpenRouter (CAI_MODEL=openrouter/z-ai/glm-4.6) — CAI has native OpenRouter support; recommended.
• Z.ai's general pay-per-token API — LiteLLM's native zai/ provider, or the generic openai/ + OPENAI_API_BASE route for models not yet in LiteLLM's registry.

It also flags the common footgun: Z.ai's Coding Plan subscription endpoints (/api/anthropic, /api/coding/paas/v4) are limited to a set of officially supported coding tools and aren't a supported path for CAI — so users don't wire a subscription key into CAI and hit throttling. It notes the OPENAI_API_KEY must-be-non-empty requirement (#27) and the parallel-tool_calls interop caveat (#469).

Changes

• docs/providers/glm.md — new provider guide.
• mkdocs.yml — registers the page under Model Providers in the nav.

Model IDs, endpoints, the zai/ prefix and the OpenRouter slugs were checked against the current Z.ai / OpenRouter / LiteLLM docs (Aug 2026); the model lists link out since they change often.

Note: the providers/openrouter.md and providers/azure.md entries already in that nav point to files that don't exist yet — pre-existing and out of scope for this PR (left untouched).