Feature: add Highlights API support (GET/POST /raindrop/{id}/highlights)

[simonives]

Summary

Raindrop.io supports highlights — annotated text passages within a saved bookmark. These are not currently exposed as tools.

Raindrop.io API

• GET /raindrop/{id}/highlights — list all highlights for a bookmark
• POST /raindrop/{id}/highlights — create a highlight (text, color, note)
• PUT /raindrop/{id}/highlights/{highlightId} — update a highlight
• DELETE /raindrop/{id}/highlights/{highlightId} — delete a highlight

API reference: https://developer.raindrop.io/v1/highlights

Proposed tools

Tool	What it does
list_highlights	Get all highlights for a bookmark
create_highlight	Add a highlight (text passage + optional note + colour)
update_highlight	Edit a highlight's text, note, or colour
delete_highlight	Remove a highlight

Implementation

New file: src/rainkeeper/tools/highlights.py following the existing pattern.
New client methods in RaindropClient under a # --- Highlights --- section.
Register in server.py.

Use case

Particularly useful for research and reading workflows where passages are annotated inside Raindrop and need to be surfaced or exported to a second brain.