fix: update MiniMax harness base URL to api.minimaxi.chat

MiniMax moved their Anthropic-compatible API from api.minimax.io to
api.minimaxi.chat. Update ANTHROPIC_BASE_URL in the minimax/v1 context
harness and corresponding comments in init.sh and the TODO note.

Model routing is unaffected: the API defaults unrecognised model names
(e.g. "minimax" from the registry slug) to MiniMax-M3 already.

Claude-Session: https://claude.ai/code/session_01XtWRazqRbMnMRa4iV3zbVx

harnesses/contexts/minimax/v1/harness.yaml

@@ -9,7 +9,7 @@ provides: [claude-code]
 # No credential env vars: the secret stays in the mounted file and is read
 # only by the helper command at request time.
 env:
-  ANTHROPIC_BASE_URL: "https://api.minimax.io/anthropic"
+  ANTHROPIC_BASE_URL: "https://api.minimaxi.chat/anthropic"
   CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS: "1"
   CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC: "1"
   DISABLE_PROMPT_CACHING: "1"
@@ -28,4 +28,4 @@ secrets_required:
 scripts:
   init: ./init.sh
 
-# TODO: Add network_hosts for api.minimax.io when context harnesses support it
+# TODO: Add network_hosts for api.minimaxi.chat when context harnesses support it

harnesses/contexts/minimax/v1/init.sh

@@ -15,7 +15,7 @@
 #
 # Auth wire-up: apiKeyHelper output is sent as `Authorization: Bearer
 # <value>` when ANTHROPIC_BASE_URL is non-anthropic.com — exactly what
-# api.minimax.io/anthropic requires. The secret value never enters this
+# api.minimaxi.chat/anthropic requires. The secret value never enters this
 # process' env, the claude subprocess' env, or /proc/<pid>/environ.
 
 set -euo pipefail