#12965: fix: suggest /clear in context overflow error message

## Summary Adds actionable recovery guidance to context overflow errors by suggesting `/clear` as an immediate fix. Closes #12940 ## Changes Updated error message from: ``` Context overflow: prompt too large for the model. Try again with less input or a larger-context model. ``` To: ``` Context overflow: prompt too large for the model. Try /clear to reset session history, or use a larger-context model. ``` ## Why - Users hitting context overflow on messaging channels (Telegram, etc.) had no clear recovery path - Gateway restarts don't help (session history persists) - `/compact` fails when context is already overflowed - `/clear` is the immediate fix that works ## Test plan - [x] `pnpm vitest run src/agents/pi-embedded-helpers.sanitizeuserfacingtext.test.ts` → 10/10 pass - [x] Updated test expectation to match new message ## AI Disclosure 🤖 Built with Claude Opus 4.5 as coding assistant. Lightly tested (unit tests pass). I understand the change: it's a simple string replacement in 3 locations with corresponding test update. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates the user-facing “context overflow” recovery text in the embedded PI agent to recommend `/clear` as the immediate remediation, and adjusts the sanitizer unit test to match. The message is emitted from both the generic assistant-error formatting path (`src/agents/pi-embedded-helpers/errors.ts`) and the embedded runner’s context-overflow recovery flow (`src/agents/pi-embedded-runner/run.ts`), ensuring users see consistent guidance when the session history itself prevents retries/compaction. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The changes are limited to updating a static, user-facing error message in three call sites plus a matching unit test expectation; no control flow or parsing logic was modified. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->