docs: add plugins page to nav, apply standards, trim design doc

- Move plugins.mdx from core-concept/ to concepts/ (matches existing
  convention; all other core-concept/ pages already migrated)
- Add to Concepts group in docs.json navigation
- Apply documentation standards: diataxis frontmatter, sentence-case
  headings, markdownlint fixes, See also footer, glossary links on
  first use of Component/Requirement/MelleaSession
- Fix broken internal link (/core-concept/interoperability → /integrations/mcp)
- Add glossary entries: Hook/HookType, Plugin, PluginSet
- Trim docs/dev/hook_system.md to internal design notes only, with
  pointer to user-facing page for usage docs

Author: planetf1