Add skills terminology glossary

[luohui1]

Summary

• Adds docs/glossary.md with short working definitions for skills-over-MCP terminology.
• Covers skill, instruction, primitive, convention, context, progressive disclosure, control model, first-class primitive, author roles, discovery, and skill resources.
• Links the glossary from the README documents table.

Fixes #51

Verification

• git diff --cached --check
• Checked README/glossary links and terminology references locally.
• Reviewed the staged diff for automation wording; none introduced.

[olaservo]

Hi @luohui1 ! A few different contributors opened glossary PRs, so I started a combined draft here: #104

Lets comment on the combined one instead (and I will add any contributors as co-authors on the commits).