Add MiniMax Anthropic-compatible workflow variants - #15

Reason: provider-add — add MiniMax Anthropic-compatible workflow variants covering MiniMax-M3 and MiniMax-M2.7.

Changes

The existing executable workflows hard-code Anthropic credentials and Claude model IDs. This adds MiniMax-backed variants that route the Claude Code CLI to MiniMax's Anthropic-compatible endpoint instead:

• code-review/claude-code-review-minimax.yml — standard code review variant.
• code-review/claude-code-review-custom-minimax.yml — pragmatic/extended code review variant.
• security-review/security-minimax.yml — automated security review variant.
• code-review/README.md and security-review/README.md — list the new templates.

How it works

• Sets ANTHROPIC_BASE_URL to the MiniMax Anthropic endpoint for the selected region (global -> https://api.minimax.io/anthropic, cn -> https://api.minimaxi.com/anthropic).
• Sets ANTHROPIC_AUTH_TOKEN from a MINIMAX_API_KEY secret so requests use Authorization: Bearer as MiniMax requires.
• Uses --model/claude-model set to MiniMax-M3 by default, switchable to MiniMax-M2.7 via a workflow_dispatch input.

Verification

• Validated all new YAML files parse with python3 -c yaml.safe_load.
• Confirmed each file references both MiniMax-M3 and MiniMax-M2.7, both regional endpoints, and the MINIMAX_API_KEY secret reference.
• No secrets, tokens, or credentials are committed; only a referenced secret name is used.