#15803: fix(skills/github): add GraphQL introspection guidance

## Summary The github skill only documents REST usage for `gh api`. When the agent needs GraphQL (e.g. GitHub Projects V2 mutations), it hallucinates incorrect mutation names, gets `undefinedField` errors, and tells the user to "use the GitHub UI." This adds: - `gh api graphql` examples (query + mutation) - Schema introspection commands to discover correct mutation/field names - Guidance to introspect on `undefinedField` errors instead of guessing ## Testing - Tested on a live OpenClaw instance (MiniMax-M2.1 via Telegram) - Before: agent hallucinated `updateProjectV2SingleSelectField` (does not exist) - After: agent used correct `updateProjectV2Field` with proper input fields ## AI Disclosure - [x] AI-assisted (Claude) - [x] Tested on live instance - [x] I understand what the change does Fixes #15796 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Updates the GitHub skill documentation to cover `gh api` GraphQL usage in addition to REST, including an example query and schema introspection snippets to avoid guessing mutation/field names when working with GitHub Projects V2. <h3>Confidence Score: 4/5</h3> - This PR is largely safe to merge, with one documentation correctness issue to fix. - Change is doc-only and guidance is generally sound, but the added GraphQL examples omit `--repo` despite the skill’s own requirement to include it outside a git directory, which can cause the documented commands to fail for users. - skills/github/SKILL.md _{Last reviewed commit: 2f26c92} <!-- 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 -->