chore: centralize Euler SDK agent skill

[Seranged]

Summary

• Removes the duplicated local euler-sdk skill copy from this repository.
• Points AGENTS.md and README.md to the canonical euler-sdk skill in euler-xyz/agent-skills.
• Keeps this repo focused on SDK implementation docs and examples while agent skill packaging lives in the central skills repo.

Changes

• Deletes skills/euler-sdk and its compiled/rule/metadata files.
• Updates AGENTS.md to load the canonical euler-sdk skill from euler-xyz/agent-skills.
• Updates README.md Agent Skills section to reference the centralized skill instructions.

Test plan

• Commit hook ran pnpm --filter @eulerxyz/euler-v2-sdk lint:fix (reported existing lint warnings; no fixes applied)
• Commit hook ran pnpm --filter @eulerxyz/euler-v2-sdk test (25 files passed, 293 tests passed)
• rg check for stale local skills/euler-sdk references in markdown