Create the engine config directory before any spawn

[sandersonstabo]

Codex login and runs failed because CODEX_HOME pointed at a directory nothing created.

🤖 Generated with Claude Code

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Summary by cubic

Fixes Codex login and runs that failed because CODEX_HOME pointed to a directory that was never created – the managed home now creates the engine config directory (.codex/.claude) before any spawn, including login.

• The engine config directory has a single definition reused for both environment variables and directory creation.
• Other engines (Grok, Cursor, OpenCode2) are unaffected; their config directories remain unmanaged.

^{Written for commit 46e4520. Summary will update on new commits.}