Fix project creation overwriting existing documents

[cursor]

Bug and impact

project_create only checked _index.json for duplicate project IDs, then unconditionally wrote projects/{id}/document.md. If an agent/user had already created that document through project_write or workspace files before metadata existed, a later project_create silently replaced the real content with the default stub or initialDocument, causing data loss.

Root cause

Project existence was defined solely by metadata in workspace/projects/_index.json; the backing document path was not checked before creation wrote to disk.

Fix

• Treat an existing projects/{id}/document.md as user data during create_project.
• Adopt the existing document into the index when no initialDocument is supplied.
• Refuse creation with initialDocument when the document already exists, directing replacement through project_write instead.
• Added regression tests for preserving an unindexed document and refusing an explicit overwrite.

Validation

• cargo +stable test --locked --manifest-path src-tauri/Cargo.toml projects::tests -- --nocapture
• cargo +stable test --locked --manifest-path src-tauri/Cargo.toml
• npm run build