#7981: fix(runner): use configured primary model as fallback default

I am using google-gemini model as my primary model, but whenever I click new session, it throws `Error: No API key found for provider "anthropic"` ## Problem Internal tools (like llm-slug-generator) calls `runEmbeddedPiAgent` without specifying a model. The runner was hardcoded to fall back to anthropic/claude-opus-4-5. If the I have no Anthropic key, it crashes. ## Solution Modified `src/agents/pi-embedded-runner/run.ts` to check `params.config.agents.defaults.model.primary` before reverting to hardcoded defaults. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR changes embedded runner default model resolution so that when `provider`/`model` aren’t provided in params, it falls back to `agents.defaults.model.primary` from config before using the hardcoded `DEFAULT_PROVIDER`/`DEFAULT_MODEL`. The logic is implemented in `src/agents/pi-embedded-runner/run.ts` prior to `resolveModel(...)`, affecting which model/provider the embedded agent uses by default. <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge, but the new precedence logic may not match the stated intent in partial-parameter cases. - Change is localized and straightforward, but the condition that only consults config primary when both params are absent could produce unexpected model selection when only one of provider/model is specified. - src/agents/pi-embedded-runner/run.ts <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} **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 -->