`thv llm setup` doesn't append provider path prefix for Envoy AI Gateway

[jerm-dro]

Problem

thv llm setup writes ANTHROPIC_BASE_URL=<gateway-url> verbatim. When the gateway is an Envoy AI Gateway, native-Anthropic traffic is served at /anthropic/v1/messages, not /v1/messages. Without the prefix, Claude Code's requests fall through to a catchall route and the user sees:

There's an issue with the selected model (claude-sonnet-4-6). It may not exist or you may not have access to it.

Gateway logs show 404, model: null, with auth headers populated — i.e. auth works, the path doesn't.

Workaround

After running thv llm setup, manually append /anthropic to the gateway URL in the patched tool config (e.g. set ANTHROPIC_BASE_URL=https://<gateway>/anthropic in Claude Code's settings).

What needs to change

thv llm setup should know that some gateways prefix provider APIs. Two pieces:

1. --anthropic-path-prefix flag (default empty). Users on Envoy AI Gateway pass --anthropic-path-prefix=/anthropic; others leave it.
2. Optional auto-probe. HEAD/OPTIONS <gateway>/anthropic/v1/messages vs /v1/messages and pick the one that returns 401 (auth-reachable) over 404.

Scope is direct-mode tools only (Claude Code, Codex). Proxy-mode tools (Cursor, VS Code) route through thv llm proxy and don't need the prefix in their config.

Acceptance

• --anthropic-path-prefix flag exists and is honored when patching ANTHROPIC_BASE_URL.
• Setup against an Envoy AI Gateway with the flag set produces a working Claude Code config.
• Setup against a non-prefixed gateway (LiteLLM, direct Anthropic) still works with the flag unset.