feat: add Zhipu AI (GLM) model provider support

Adds zhipuai as a new provider using the OpenAI-compatible API at
api.z.ai. Supports api_key config or ZHIPUAI_API_KEY env var, with
optional endpoint override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to Flynn are documented in this file.
 
 ### Added
 
+- **Zhipu AI (GLM) Provider** -- Support for Zhipu AI's GLM models (glm-4.5, glm-4.7, etc.)
+  via their OpenAI-compatible API at `https://api.z.ai/api/paas/v4`. Uses `provider: zhipuai`
+  in config with `api_key` or `ZHIPUAI_API_KEY` env var.
+
 - **Agent Tool: file.patch** -- Multi-file, multi-hunk structured patch tool. Apply
   line-based replacements, insertions, and deletions across multiple files in a single
   tool call. Hunks are applied bottom-up to preserve line numbers. 10 tests.