#15749: fix: improve context overflow error with diagnostic details

## Summary - Replaces the generic "Context overflow: prompt too large" message with diagnostic details - Now includes provider/model name, context window limit, session identifier, and message count - Adds actionable recovery steps (run /reset, switch model, adjust contextTokens) **Before:** ``` Context overflow: prompt too large for the model. Try /reset (or /new) to start a fresh session, or use a larger-context model. ``` **After:** ``` Context overflow (anthropic/claude-opus-4-5, limit: 200,000 tokens) Session: discord:channel:123456789 Messages: 47 Actions: • Run /reset to clear context and start fresh • Or switch to a model with a larger context window • Consider lowering contextTokens in config to trigger earlier compaction ``` Closes #9409 ## Test plan - [x] Lint passes (oxlint, no warnings/errors) - [x] Existing e2e tests check `isError: true` and `error.kind`, not exact text — unchanged - [ ] Manual test: trigger a context overflow and verify the new message format 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change updates the embedded PI agent’s context-overflow handling to return a multi-line error message with diagnostics (provider/model, context-window limit, session label, message count) and suggested recovery actions instead of the previous generic one-liner. The behavior lives in `src/agents/pi-embedded-runner/run.ts` inside the overflow/compaction recovery branch, and affects what end users see when the runner detects a context overflow and can’t recover via auto-compaction/tool-result truncation. <h3>Confidence Score: 3/5</h3> - This PR is close to safe to merge but has a couple of user-facing error-handling edge cases to fix first. - Core change is localized and straightforward, but the new formatting can throw if the context limit isn’t a number, and it may expose sensitive session identifiers and mislabel compaction failures as context overflows. - src/agents/pi-embedded-runner/run.ts _{Last reviewed commit: 281f021} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->