#7208: Capture tool error occurredAtMs at tool-result boundary

### Why Review note: `occurredAtMs` should reflect the earliest possible timestamp at the boundary where the tool result event is received, not after sanitization/processing. ### What - Capture `occurredAtMs = Date.now()` at the start of `handleToolExecutionEnd` and propagate it into `lastToolError` when a tool fails. - Add a short comment clarifying this is handler receipt time (pi-agent-core events do not currently include timestamps). ### Scope Small/low-risk; improves timestamp fidelity without changing any external tool result contracts. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR captures a best-effort `occurredAtMs` timestamp at the top of `handleToolExecutionEnd` and stores it on `state.lastToolError` when a tool call fails. The intent is to make the error timestamp reflect handler receipt time (tool-result event boundary) rather than later processing/sanitization time, since `pi-agent-core` events currently lack native timestamps. The change is localized to the embedded Pi tool event handler and does not alter the emitted tool-result event contract (only internal error tracking gains the timestamp). <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is small, localized, and does not modify outward-facing tool event payloads; it only captures `Date.now()` earlier and threads it into an internal `lastToolError` record. No control flow changes beyond adding a timestamp field. - No files require special attention <!-- 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 -->