#5945: fix: use configured model for slug generator (AI-assisted)

## Summary - Use the configured default agent model for LLM slug generation instead of falling back to Anthropic defaults. - Prevent /new hook failures in setups that only configure non-Anthropic providers (e.g., google-antigravity + google-gemini-cli). ## Details Slug generation uses a temporary embedded agent without specifying provider/model, which defaults to Anthropic. In environments without Anthropic auth, /new triggers FailoverError and can disrupt message handling. This change resolves the default model from config and passes provider/model into the embedded run. ## Testing - Not run (local reproduction from a live server setup). ## AI Assistance AI-assisted (OpenCode). Prompt history available on request. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the LLM-based slug generator hook to use the default model configured for the resolved default agent, instead of implicitly falling back to the global Anthropic defaults. Concretely, `src/hooks/llm-slug-generator.ts` now calls `resolveDefaultModelForAgent({ cfg, agentId })` and passes the returned `{ provider, model }` into `runEmbeddedPiAgent`, preventing failures in environments where Anthropic is not configured but other providers are. This fits existing patterns in the codebase: other embedded/agent flows resolve a model ref from config (with agent overrides) before running, while the embedded runner itself still retains a final fallback to `DEFAULT_PROVIDER/DEFAULT_MODEL` if nothing is provided. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is narrowly scoped (one hook), follows existing model-selection utilities, and simply plumbs an already-supported `provider/model` pair into `runEmbeddedPiAgent` to avoid unintended Anthropic defaults. No behavior changes outside slug generation and no new side effects beyond selecting the configured default model. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->