fix(delegate): recognise MiniMax /anthropic endpoint in delegation

[MKheru]

Bug

When `delegation.base_url` is configured to MiniMax's Anthropic-compatible endpoint (`https://api.minimax.io/anthropic\` or `https://api.minimaxi.com/anthropic\`), `_resolve_delegation_credentials` falls back to:

```
provider=custom api_mode=chat_completions
```

Sub-agents then POST to `/v1/chat/completions` (OpenAI format) which doesn't exist on MiniMax → `HTTP 404: 404 page not found` after 3 retries.

Symptom

GPT-5.5 main agent (Codex provider) tries to delegate research tasks to MiniMax-M2.7 children via `delegate_task`. Every child fails:

```
[subagent-0] API call failed after 3 retries. HTTP 404: 404 page not found | provider=custom model=MiniMax-M2.7
```

Confirmed during WS9 GPT-5.5 evaluation 2026-05-04 ~13:55 UTC. Fallback path forced GPT-5.5 to do all 5 sub-tasks itself.

Fix

Add a 4th branch to the URL auto-detection in `_resolve_delegation_credentials` mapping `api.minimax.io` / `api.minimaxi.com` with `/anthropic` in the path to `provider=minimax` + `api_mode=anthropic_messages`. The child then builds a correct `POST /v1/messages` call at `/anthropic/v1/messages`.

10 lines added, no logic modified for existing branches.

Verification

After deploying the fix on acos-hermes-01, re-ran WS9 GPT-5.5 evaluation:

• Batch 1 (SR1/SR2/SR3) → all 3 children completed via MiniMax
• Batch 2 (SR4/SR5) → both completed
• GPT-5.5 rejected SR3 + SR5 partially and re-delegated with targeted briefs (the orchestration test we wanted to run originally)
• Total: 64 MiniMax API calls, ~27min wall-clock cumulative
• Final livrable: `~/WS9_GUARDIAN_DESIGN_DEEP.md`, 412 lines

Test plan

• Unit syntax check on `tools/delegate_tool.py`
• Restart hermes-agent on VPS, no boot error
• Live test: GPT-5.5 successfully delegates 5+ sub-tasks to MiniMax children
• No regression on Codex/Anthropic/Kimi paths (only added a new `elif` branch)

Notes

This fix is independent of #6 (WS-AUTO-002 AFK Work Loop). No conflict expected at merge time.