Add agent skill docs (CLAUDE.md) for plotlink-ows API

[realproject7]

Problem

When an agent tries to use plotlink-ows features (e.g., update story metadata, upload cover image), there are no skill definitions, CLAUDE.md, or API docs available. The agent has to reverse-engineer endpoints by reading installed package source code under ~/.npm/_npx/.

From a real session — the agent had to:

1. Hit /api/health to confirm the server was running
2. Find the login endpoint by trial
3. Grep through app/routes/ in the npm cache to find route definitions
4. Read publish.ts to get exact endpoint signatures

This took over 2 minutes of research for what should be a 5-second lookup.

Scope

Update CLAUDE.md at the package root to document the local API:

Auth

• How to authenticate (login endpoint, bearer token)
• Where the OWS passphrase lives

API Endpoints

• POST /api/publish/upload-cover — file upload (form-data, max 500KB, WebP/JPEG), returns { cid }
• POST /api/publish/update-storyline — fields: storylineId (required), coverCid, genre, language, isNsfw
• POST /api/publish — full publish flow (storyline + plot)
• GET /api/stories — list stories
• Any other routes in app/routes/

Available genres / languages

• Reference to the valid values

Common workflows

• "Update cover image for story #N" (upload-cover → update-storyline)
• "Set genre and language for story #N"
• "Publish a new story"

Blocker

• [Bug] Cover image upload fails with 'Invalid message format' #180 — cover upload is broken due to \r\n newline corruption in multipart form-data. Skill docs should note this limitation until fixed.

Acceptance Criteria

• CLAUDE.md updated with local API endpoint docs
• Agent can discover and call endpoints without reading source code
• Covers auth, all publish-related endpoints, and common workflows
• Notes [Bug] Cover image upload fails with 'Invalid message format' #180 blocker for cover upload

Refs: #180