docs(agent-skill): fix stale repo references

[WadydX]

Summary

• replace dead docs/README.md skill reference with docs/index.md
• drop the stale README.md#security reference
• update auth-related repo paths to the current packages

Why

The bundled gog agent skill currently points at repo paths that do not exist:

• docs/README.md
• README.md#security
• internal/auth/

This updates the skill to reference files/directories that exist in the current repo layout.

Closes #557

[steipete]

Landed on main as 25bae5c.\n\nI merged current main into the PR branch, added changelog credit, and verified with path checks plus:\n- make ci\n- git diff --check\n\nThanks @WadydX.