feat(aifeed): add the AIFeed signed content permissions plugin

[denyn1]

What this solves

Agents fetching the web cannot check whether a site's permission rules are authentic, current, or revocable: robots.txt and llms.txt are unsigned text. AIFeed is an open standard (MIT code, CC BY 4.0 specs) where a site publishes /.well-known/ai.json signed with Ed25519, anchors the key in a DNS _aifeed TXT record, and can revoke it; agents verify the chain before fetching.

This plugin gives ZCode that workflow:

• MCP server (aifeed, stdio) with six tools: verify_manifest, fetch_aifeed, list_assets, verify_asset, select_index, and decide_usage.
• Skill (aifeed) with verify-first and publish flows, so the agent checks permissions before fetching and never assumes allow.

Testing

• python scripts/validate.py -> OK: 27 plugin(s) validated
• python scripts/build_dist.py -> builds plugins/aifeed/1.0.0-draft.1/plugin.zip
• git diff --check clean
• Ran the bundled MCP server over stdio from the plugin directory: initialize returns serverInfo aifeed-mcp-server (protocol 2025-06-18) and tools/list returns the six tools. The same engine is covered by the upstream test suite (283 JS + 56 Python tests) at https://github.com/denyn1/aifeed-protocol.
• No UI changes, so no screenshots; the verification steps above are reproducible.

Version and registration

• New plugin aifeed, version 1.0.0-draft.1, registered in marketplace.json under category developer-tools; the manifest and the marketplace entry match (checked by validate.py).
• The draft version mirrors the AIFeed spec line (1.0.0-draft), the same versioning used for the project's npm and PyPI packages.

Dependencies, network, permissions, and side effects

• Zero runtime dependencies: the stdio server is bundled under engine/ and uses the Node standard library only. Node 20+ (node on PATH) is required; there is no install step at runtime.
• Network: HTTPS requests to the domain the user asks about (manifest, content, assets, DNS TXT lookups). No telemetry; nothing is sent to the author.
• Writes: none. The plugin never writes to the project, the home directory, or ZCode data.
• http:// loopback origins are allowed only when the allow_private user option is set to 1 (local testing).
• No hooks and no commands; one skill plus one MCP server.

Licensing and provenance

• Plugin code and the bundled engine are MIT, from https://github.com/denyn1/aifeed-protocol (packages/aifeed-mcp-server, vendored as a generated CJS copy under engine/). The LICENSE file is included.
• No third-party assets. The icon URL points at the project's own deployed logo (https://aifeed.md/logo.svg).

Checklist

• the plugin name is unique and kebab-case (aifeed)
• required files and both language READMEs are present (README.md, README_CN.md)
• the category is correct (developer-tools)
• no secrets, private data, or machine-specific paths are included
• validate.py, build_dist.py, and git diff --check pass
• review feedback will be addressed on this pull request