#15860: docs(github): add GraphQL API guidance and schema introspection

## Problem The GitHub skill only documents REST API usage, causing agents to hallucinate incorrect GraphQL mutation names. ## Solution Added GraphQL documentation including schema introspection and troubleshooting. Fixes #15796 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR expands the GitHub skill documentation to cover GraphQL usage (including schema introspection and error troubleshooting) so agents can discover correct mutation/field names instead of guessing. It also tweaks the compaction docs configuration section and updates the Moonshot provider’s default model ID (plus adds several additional Moonshot model definitions) to match the expected API model naming. Overall, the changes are additive and aligned with existing patterns (skill docs under `skills/github/`, provider defaults under `src/agents/models-config.providers.ts`). The main follow-ups are around keeping docs navigable (don’t remove the only link to compaction config) and ensuring the new GraphQL CLI examples are copy/paste reliable. <h3>Confidence Score: 4/5</h3> - This PR is generally safe to merge; remaining concerns are limited to documentation accuracy/usability. - Changes are mostly documentation plus a small provider default update. No clear runtime-breaking logic was introduced, but a docs self-reference was removed and one GraphQL example is brittle to copy/paste, which could mislead users/agents. - docs/concepts/compaction.md, skills/github/SKILL.md _{Last reviewed commit: 2d5eb50} <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->