feat(ai-review): add agent LLM provider backed by local Claude Code

[vineethkrishnan]

What

Adds a sixth LLM_PROVIDER value, agent, that routes reviews through a local Claude Code agent exposing POST /trigger (which runs claude -p non-interactively) instead of calling an LLM API directly.

Why

When ClearPR runs in Docker, it cannot invoke the host's claude binary or read its credentials. Pointing the new provider at a small host-side agent lets a deployment reuse an existing Claude subscription on the host, so no Anthropic API key is needed.

How

• AgentLlmAdapter POSTs the prompt with Authorization: Bearer <LLM_API_KEY>, maps the agent's { result.result, usage, modelUsage } payload onto LlmResponse, applies a 600s timeout matching the agent's own ceiling, and surfaces aborts as LlmTimeoutError and bad payloads as MalformedLlmResponseError.
• Wired into LlmProvider enum, llm-provider.registry.ts, and the default-model map (claude-code).
• docker-compose.yml gains host.docker.internal:host-gateway so the container can reach a host-side agent.
• README and the provider guide document the new option.

Config

LLM_PROVIDER=agent
LLM_BASE_URL=http://host.docker.internal:8765
LLM_API_KEY=<agent bearer token>

Tests

6 new unit tests for the adapter (success mapping, model fallback, error result, non-2xx, abort to timeout). Full suite 170/170, tsc/eslint/nest build clean.