Split PR: add filesystem APIs on top of exec changes

[starr-openai]

Summary

• Adds filesystem-related API wiring and session/environment plumbing from Add the exec-server end-to-end stack #15069.

Includes

• commits: 5285a87 (Inject executor-aware filesystem into app-server)
• 3b64c91 (Load remote repo skills through session environment)
• 462404b (Route remote project docs and repo skills through executor environment)
• branch: starr/exec-server-split-pr3-fs

[github-actions]

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