feat(provider): add MiniMax regional support

[octo-patch]

Reason: Add a first-class MiniMax provider with current models and regional endpoint selection.

This change:

• adds MiniMax-M3 and MiniMax-M2.7 with their current context windows
• selects the global or China endpoint through MINIMAX_REGION, with an explicit URL override available
• registers the provider in the runtime selector and documents its environment configuration
• prevents the MiniMax credential from reaching local CLI subprocesses and recognizes it in credential-exfiltration scans
• adds focused coverage for credentials, both regions, endpoint overrides, model metadata, chat construction, selection, and credential scrubbing

Checks:

• uv run pytest -q tests/unit/test_providers.py tests/unit/test_minimax_provider.py
• uv run pytest -q tests/unit/test_agent_cli.py
• uv run ruff check src/skillspector/providers/minimax tests/unit/test_minimax_provider.py src/skillspector/providers/__init__.py src/skillspector/providers/_agent_cli.py src/skillspector/cli.py
• uv run ruff format --check src/skillspector/providers/minimax tests/unit/test_minimax_provider.py src/skillspector/providers/__init__.py src/skillspector/providers/_agent_cli.py src/skillspector/cli.py
• uv run mypy src/skillspector/providers/minimax
• uv run python -m build