installer: failed install leaves a partially managed workspace

[LeonEvo1103]

Summary

An install that fails on a later file conflict leaves earlier managed files written to the workspace, producing a partial installation.

Verified on main at 3048fb73910a0a8944f74fb2e70abd5752a60b2f.

Reproduction

1. Create a temporary workspace.

2. Pre-create a user-owned .vscode/mcp.json without the Evolver managed marker.

3. Run:

   node scripts/install.js --install --workspace <temporary-workspace>
   

Actual result

The command exits 1 and correctly refuses to overwrite .vscode/mcp.json, but it has already written:

• managed .github/copilot-instructions.md
• all eight managed .github/prompts/*.md files

The runtime directories are not installed yet, leaving an inconsistent half-installed workspace.

Expected result

Either preflight every destination before writing, or roll back files written by the failed invocation.

Cause

scripts/install.js:196-212 writes instructions and prompts immediately. The MCP conflict is not checked until lines 214-216, and there is no transaction or rollback.

Impact

An installation reported as failed still mutates the workspace and requires cleanup before retrying.