#12688: docs: add troubleshooting guide for thinking.signature error

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a new troubleshooting doc (`docs/debug/thinking-signature-error.md`) explaining the `thinking.signature: Field required` failure mode and how to recover (delete or manually edit the corrupted session `.jsonl`) when external AI messages are written into the session history. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge after correcting a misleading field name in the troubleshooting instructions. - The change is documentation-only and limited in scope, but the current manual-edit recovery steps reference a different key (`thoughtSignature`) than the actual error path (`thinking.signature`), which would directly prevent users from following the fix. - docs/debug/thinking-signature-error.md <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **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 -->