feat: add MiniMax M2.5 as a provider option

[octo-patch]

Summary

Adds MiniMax as a fourth AI provider alongside Claude, Gemini, and OpenRouter.

• New MiniMaxAgent using OpenAI-compatible REST API via fetch(), modeled after OpenRouterAgent
• Supports models MiniMax-M2.5 (default) and MiniMax-M2.5-highspeed with 204K token context
• Multi-turn conversation, context truncation, token tracking, and fallback to Claude on retriable errors (429/500)
• Settings: CLAUDE_MEM_MINIMAX_API_KEY, CLAUDE_MEM_MINIMAX_MODEL, CLAUDE_MEM_MINIMAX_BASE_URL, CLAUDE_MEM_MINIMAX_MAX_CONTEXT_MESSAGES, CLAUDE_MEM_MINIMAX_MAX_TOKENS
• Credential isolation via ~/.claude-mem/.env (MINIMAX_API_KEY)
• Provider selection integrated in worker-service.ts, SessionRoutes.ts, SettingsRoutes.ts

MiniMax API constraints handled

• Temperature defaults to 1.0 (MiniMax requires (0.0, 1.0], cannot be 0)
• Default base URL: https://api.minimax.io/v1 (overseas)
• No response_format support (uses prompt engineering + XML parsing)

Files changed (8)

File	Change
src/services/worker/MiniMaxAgent.ts	New provider implementation
tests/minimax_agent.test.ts	6 unit tests (all passing)
src/services/worker-service.ts	Provider initialization & fallback chain
src/services/worker-types.ts	Added 'minimax' to provider union type
src/services/worker/http/routes/SessionRoutes.ts	Provider selection logic
src/services/worker/http/routes/SettingsRoutes.ts	Valid provider list
src/shared/EnvManager.ts	MINIMAX_API_KEY credential management
src/shared/SettingsDefaultsManager.ts	MiniMax config defaults

Test plan

• Unit tests pass (6/6) via bun test tests/minimax_agent.test.ts
• Set CLAUDE_MEM_PROVIDER=minimax and CLAUDE_MEM_MINIMAX_API_KEY in settings
• Verify MiniMax provider is selectable and processes observations
• Verify fallback to Claude on MiniMax API errors (429/500)
• Verify non-retriable errors (401) are thrown without fallback