Streamline AGENTS.md and fix outdated content

This commit reduces AGENTS.md by ~51% (650 → 321 lines) to minimize
token usage for LLM agents while maintaining all essential information.

Key improvements:

- Removed duplicate content by referencing CONTRIBUTING.md for branch
  policy, commit conventions, bugfix/feature processes, and changelog
  formatting
- Condensed repository structure section to reference README.md for
  the complete package list, keeping only deprecated path warnings
- Moved VitePress documentation guidelines from AGENTS.md to
  docs/README.md where they belong
- Removed verbose Markdown conventions that are now automated by
  Hongdown

Outdated content fixes:

- Updated testing framework references: now uses @fedify/fixture
  (runtime-agnostic test adapters) instead of "Deno's testing
  framework"
- Corrected ActivityPub vocabulary location: packages/vocab/vocab/
  (not packages/fedify/src/vocab/)
- Fixed @fedify/cli description: now tested with Deno, Node.js, and
  Bun (not Deno-only), uses deno compile for executables
- Removed non-existent src/codegen/ directory reference from
  CONTRIBUTING.md
- Added missing internal packages to CONTRIBUTING.md: @fedify/vocab,
  @fedify/webfinger, @fedify/fixture

[ci skip]

Author: dahlia

.hongdown.toml

@@ -3,6 +3,7 @@ include = ["*.md", "**/*.md"]
 exclude = [
   "**/node_modules/**",
   "**/dist/**",
+  ".github/copilot-instructions.md",
   "AGENT.md",
   "CLAUDE.md",
   "GEMINI.md",