fix(project): map Windows drive-root basename to unknown in MCP detection

[dnlrsls]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

📝 Bug Description

On Windows, starting engram mcp with the process current working directory at a drive root (C:\) causes Go project auto-detection to return a backslash as the project name. That value is rejected when supplied explicitly to mem_save, creating an inconsistent MCP contract.

🔄 Steps to Reproduce

1. On Windows, start engram mcp with process cwd C:\.
2. Call mem_current_project from an OpenCode MCP client.
3. Observe the returned project value, then pass it as project to mem_save.

✅ Expected Behavior

Auto-detection must never return a project value that explicit project validation rejects. For a Windows drive root without Git/config detection, return project: "unknown" while retaining project_source: "dir_basename".

❌ Actual Behavior

mem_current_project returns:

{ "cwd":"C:\\", "project":"\\", "project_path":"C:\\", "project_source":"dir_basename" }

Passing project: "\\" to mem_save returns error_code: invalid_project, because a project must be a name rather than a path.

Operating System

Windows

Engram Version

v1.20.1-0.20260720141607-763a6ba43271 (commit 763a6ba432713725d6ce82a2416eec6cbd9ec94e)

Agent / Client

OpenCode

📋 Relevant Logs

mem_current_project -> { "cwd":"C:\\", "project":"\\", "project_path":"C:\\", "project_source":"dir_basename" }
mem_save(project: "\\") -> error_code: invalid_project

💡 Additional Context

Environment: Windows, OpenCode MCP client, PowerShell 7. The current template has no shell field; if one is added, select Other.

Root cause: internal/project/detect.go in DetectProjectFull Case 5 uses filepath.Base(dir) and maps only "" and "." to "unknown". On Windows, filepath.Base("C:\\") == "\\". internal/mcp/mcp.go (handleCurrentProject) returns the detection result verbatim. Explicit project validation correctly rejects path separators, while omitted-project write resolution can accept the auto-detected backslash.

Suggested tests:

• Windows drive root in DetectProjectFull.
• mem_current_project never emits a separator as project.
• Explicit backslash remains rejected.

Related but not duplicates: #652 and #656 address separator handling in the OpenCode TypeScript plugin, not the Go detector.