#14556: fix(cron): exclude sandbox from shallow merge in isolated agent config

## Summary - **Bug**: When a per-agent `sandbox` config contains only `docker` overrides (e.g. `workdir`, `network`, `binds`), the `Object.assign` shallow merge in `runCronIsolatedAgentTurn` replaces the entire `agents.defaults.sandbox` — losing `mode`, `scope`, and `workspaceAccess`. This causes `resolveSandboxConfigForAgent` to fall back to `mode: "off"`, resulting in cron isolated agents executing directly on the **host** instead of inside the sandbox container. - **Fix**: Exclude `sandbox` from the destructured agent config override. Sandbox resolution is already handled separately by `resolveSandboxConfigForAgent`, which properly merges global defaults with per-agent overrides via `agents.list[].sandbox`. ## Reproduction 1. Set `agents.defaults.sandbox.mode: "all"` and `scope: "agent"` 2. Add a per-agent `sandbox.docker` override (e.g. custom `workdir`, `network`, or `binds`) in `agents.list[]` 3. Create a cron job with `sessionTarget: "isolated"` for that agent 4. Run the cron job and execute `hostname && whoami` — it prints the **host** hostname and user instead of the container's ## Root cause ``` src/cron/isolated-agent/run.ts:130-136 Object.assign({}, params.cfg.agents.defaults, // sandbox: { mode:"all", scope:"agent", ... } agentOverrideRest, // sandbox: { docker: { ... } } ← overwrites entirely ) // Result: sandbox.mode is gone → defaults to "off" ``` Related: #4171 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates the cron isolated-agent runner to avoid shallow-merging a per-agent `sandbox` override into `agents.defaults`. Previously, `runCronIsolatedAgentTurn` built `cfgWithAgentDefaults` by doing a top-level `Object.assign({}, agents.defaults, agentOverrideRest)`. If the per-agent override included only `sandbox.docker` keys, that shallow merge replaced the entire `sandbox` object and dropped `mode/scope/workspaceAccess`, causing downstream sandbox resolution to fall back to `mode: "off"` and run cron “isolated” turns on the host. The fix destructures `sandbox` out of the per-agent override before the shallow merge, relying on the existing `resolveSandboxConfigForAgent` path (which merges `agents.defaults.sandbox` with `agents.list[].sandbox` field-by-field) to handle sandbox configuration correctly. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The commit is narrowly scoped (excludes `sandbox` from a shallow merge) and aligns with the existing sandbox resolution flow (`resolveSandboxConfigForAgent` merges global defaults with per-agent overrides). No other files are touched in the head SHA, and the change prevents a real configuration regression without affecting unrelated agent defaults. - src/cron/isolated-agent/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!} <!-- /greptile_comment -->