engine/codergen: seed codex auth when HOME is unset

[danshapiro]

Summary

• fix codex home discovery for auth/config seeding to work when HOME is unset (common in Windows PowerShell)
• add fallbacks to USERPROFILE, HOMEDRIVE+HOMEPATH, and os.UserHomeDir()
• use the same fallback in codexStateBaseRoot() so isolated codex state defaults under the real user home instead of a relative run directory
• add regression tests for USERPROFILE fallback seeding and state-base fallback

Validation

• go test ./internal/attractor/engine -run "TestBuildCodexIsolatedEnv_SeedsFromUserProfileWhenHomeUnset|TestCodexStateBaseRoot_FallsBackToUserProfileWhenHomeUnset"
• go test ./internal/attractor/engine -run "TestCodexStateBaseRoot_FallsBackToUserProfileWhenHomeUnset|TestCodexCLIInvocation_StateRootIsAbsolute"

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Gate Windows home fallbacks to Windows hosts

codexSourceHome always considers HOMEDRIVE/HOMEPATH, even on Linux/macOS. When HOME and USERPROFILE are unset but those Windows vars are present (for example in mixed-shell or WSL-launched environments), windowsHomeFromParts can return C:\Users\..., which is not an absolute Unix path; this candidate is selected before os.UserHomeDir(), so codexStateBaseRoot() can resolve state under the current working directory with a literal C:\... segment and auth/config seeding will target the wrong directory.

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