feat: add OpenRouter as a first-class LLM provider

[octo-patch]

Fixes #612

Problem

OpenRouter is a popular API gateway that provides unified access to hundreds of models from providers like OpenAI, Anthropic, Google, and others via an OpenAI-compatible API. Previously, users had to configure it as an unsupported provider by manually specifying base_url and provider_interface, which was undocumented and fragile.

Solution

Add openrouter as a first-class supported provider:

• crates/hermesllm/src/providers/id.rs: Add OpenRouter variant to ProviderId enum with TryFrom<&str> mapping and OpenAI-compatible API routing
• crates/common/src/configuration.rs: Add OpenRouter variant to LlmProviderType enum
• cli/planoai/config_generator.py: Add "openrouter" to SUPPORTED_PROVIDERS_WITH_BASE_URL (requires base_url since the endpoint is external)
• config/plano_config_schema.yaml: Add "openrouter" to the provider_interface enum

Users can now configure OpenRouter like this:

model_providers:
  - model: openrouter/openai/gpt-4o
    access_key: $OPENROUTER_API_KEY
    base_url: https://openrouter.ai/api/v1
    default: true
  - model: openrouter/anthropic/claude-3-5-sonnet
    access_key: $OPENROUTER_API_KEY
    base_url: https://openrouter.ai/api/v1

OpenRouter uses the OpenAI-compatible chat completions API, so no new request/response types are needed.

Testing

• All existing Rust tests pass (cargo test --lib -p hermesllm -p common)
• Verified cargo check succeeds for all modified crates