Expose cross-post toggles (LinkedIn, X, Threads, Bluesky) on MCP note tools

Add per-note distribution toggle fields — starting with crossPostToLinkedIn — to the following MCP tools: create_note, create_many_notes, update_note, schedule_notes, and schedule_notes_with_custom_dates.

Currently there's no way to set or toggle LinkedIn cross-posting via the MCP — it's UI-only, requiring users to go note by note manually.

Two key use cases:

1. Default cross-post ON for an entire weekly batch scheduled in bulk

2. Selectively enable cross-post only for certain note types (e.g. Value notes) while leaving others off — since LinkedIn doesn't render Substack URLs well, Soft Promo notes shouldn't cross-post

A single crossPostToLinkedIn: true/false boolean alongside existing fields like autoRestackAfterHours would solve this. Support for X, Threads, and Bluesky toggles would be a natural extension.

Reported by Phil Powis (Sacred Business Flow, ~27k subscribers) — using WriteStack MCP to bulk-schedule notes from Claude and hitting this as the highest-friction missing piece in the workflow.