Add collaborative markdown document tools

[samwillis]

Summary

Adds collaborative markdown document support for Electric Agents, backed by Yjs durable streams.

• Adds runtime markdown tools for creating, reading, replacing, editing, cursor placement, and streaming inserts into markdown docs.
• Keeps document content edits local to the agent runtime by materializing a wake-local Y.Doc and appending framed binary Yjs updates to the document stream.
• Adds streaming tool-argument handling so insert_markdown_doc chunks are applied and appended as arguments stream in.
• Uses Yjs relative positions for stateful Horton insertion cursors.
• Thins server-side markdown handling to stream creation, manifest metadata, and raw Yjs stream proxying; semantic server edit/write endpoints are removed.
• Updates Horton prompt/skill guidance and adds changesets for the touched packages.

Validation

• pnpm --filter @electric-ax/agents-runtime typecheck
• pnpm --filter @electric-ax/agents-runtime exec vitest run test/markdown-docs-tools.test.ts
• pnpm --filter @electric-ax/agents-server typecheck
• pnpm --filter @electric-ax/agents-server exec vitest run test/electric-agents-routes.test.ts test/electric-agents-manager-write-validation.test.ts
• pnpm --filter @electric-ax/agents-server-ui typecheck
• pnpm --filter @electric-ax/agents-server-ui exec vitest run src/components/views/MarkdownDocumentView.test.ts
• pnpm --filter @electric-ax/agents typecheck
• pnpm --filter @electric-ax/agents exec vitest run test/horton-tool-composition.test.ts test/horton-system-prompt.test.ts
• pnpm --filter @electric-ax/agents-desktop typecheck
• git diff --check

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