#9429: fix: skip session model override for heartbeat runs

Fixes #9375 ## Problem When `agents.defaults.heartbeat.model` is set (e.g., `google/gemini-2.5-pro`), heartbeat runs should use that model. However, if the main session has a `modelOverride` stored (e.g., from using `/model opus` command), the heartbeat incorrectly runs on the session's overridden model instead. ## Root Cause In `createModelSelectionState()`, the session's stored override takes precedence and overwrites the heartbeat model that was already resolved in `get-reply.ts`. ## Solution 1. Added `isHeartbeat` parameter to `createModelSelectionState()` 2. Skip the `storedOverride` logic when `isHeartbeat` is true 3. Pass `opts?.isHeartbeat` from `resolveReplyDirectives()` Now `heartbeat.model` configuration takes precedence for heartbeat runs, as expected. ## Testing - Verified heartbeat runs use configured heartbeat model even when session has a different model override - Verified normal chat messages still respect session model overrides <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR threads an `isHeartbeat` flag into model selection, and when set it prevents session/parent stored model overrides (e.g., from `/model`) from overwriting the already-resolved heartbeat model. The change is applied in `resolveReplyDirectives()` (passing `opts?.isHeartbeat`) and in `createModelSelectionState()` (skipping `resolveStoredModelOverride()` application). <h3>Confidence Score: 4/5</h3> - Mostly safe to merge, but heartbeat runs may still incur stored-override side effects. - The core precedence fix is narrowly scoped and appears correct, but `createModelSelectionState()` still treats stored overrides as present for catalog-loading and override-reset logic even on heartbeats, which could cause unnecessary work and unexpected session mutations during heartbeat runs. - src/auto-reply/reply/model-selection.ts <!-- greptile_other_comments_section --> **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 -->