fix: add plugin marketplace manifest so the plugin can be installed

[917Dhj]

Problem

The repository ships .claude-plugin/plugin.json but no .claude-plugin/marketplace.json, so installing it through Claude Code's own plugin system fails:

claude plugin marketplace add 917Dhj/DeepPaperNote
→ Failed to add marketplace: Marketplace file not found at
  .../.claude-plugin/marketplace.json

This was not a never-written file — git log shows the manifest was added in b3ac109, removed in e49fe61 (the multi-agent generality pass), and never restored when plugin.json came back in 1afbfc3. So the repository advertises a plugin identity it cannot actually deliver.

Change

Add .claude-plugin/marketplace.json (16 lines) and document the resulting second install route.

• The plugin entry uses source: "./" to point back at the repository root.
• The entry deliberately omits version, so plugin.json remains the single version source. The previous deleted entry hardcoded 0.1.0 in both places, which drifts; version resolution gives plugin.json precedence anyway.
• READMEs now present both routes: claude plugin marketplace add for Claude Code, npx skills add for any agent.
• AGENTS.md repository-structure list mentions the new manifest; changelog records the new install path per the repo's own "changes how users install" convention.

Verification

• claude plugin validate . --strict → exit 0, no errors, no warnings. (The previous CLAUDE.md at the plugin root is not loaded as project context warning also disappears, because validation now reads the marketplace manifest rather than treating the repository root as a plugin root.)
• End-to-end against a local path in an isolated config dir: marketplace add → plugin install deeppapernote@deeppapernote → Skills (2) deeppapernote, paper-glossary load correctly.
• python3 -m pytest -q → 923 passed, 1 skipped.
• The npx skills add route still discovers both skills with and without .claude-plugin/, so nothing regresses for the agent-neutral channel.

Follow-up

Once this merges, claude plugin marketplace add 917Dhj/DeepPaperNote becomes usable. Note that future releases must bump version in plugin.json, otherwise claude plugin update skips silently.

🤖 Generated with Claude Code