Document memory MCP tools

[rsd-raindrop]

Summary

• Documents the new Raindrop Memory MCP tools in the public investigate skill reference.
• Covers list/create/update/delete behavior, categories, project scoping, and the write:memories OAuth scope.

Validation

• Documentation-only change; reviewed rendered Markdown structure locally.

Risk Assessment

• Scope: raindrop-investigate MCP tool reference docs.
• Breaking changes: None.
• Rollback: Yes. Simple revert removes the new Memory section.
• Confidence: High. Mirrors the MCP tool names and inputs from the Dawn implementation.