thv llm: package skeleton, cobra commands, and config management

[yrobla]

Context

Part of #5016 (RFC: stacklok/toolhive-rfcs#70). This is the foundation subtask — all other subtasks depend on this.

Scope

• pkg/llm/config.go — config types (LLMConfig, gateway URL, OIDC settings, proxy port)
• pkg/llm/internal/ — package structure scaffolding
• Wire thv llm cobra command group into root command with subcommand stubs
• thv llm config set — persist gateway URL, OIDC settings, proxy port to config.yaml under llm: key
• thv llm config show — display current LLM config; --output json produces valid JSON
• thv llm config reset — clear all LLM config and delete cached tokens from secrets provider

Acceptance Criteria

• thv llm config set --gateway-url <url> --issuer <issuer> --client-id <id> persists settings to config.yaml under llm:
• thv llm config show displays current config; --output json produces valid JSON
• thv llm config reset clears LLM config and removes cached tokens from the secrets provider
• thv llm subcommand group is reachable from the root command; all subcommands are stubbed and return a not-implemented error until filled in by subsequent subtasks

Dependencies

None — this is the first subtask.

References

• Parent issue: thv llm subcommand for LLM gateway authentication #5016
• RFC: RFC-0070: thv llm subcommand for LLM gateway authentication toolhive-rfcs#70