Add the exec-server end-to-end stack

[starr-openai]

Summary

• add the exec-server crate and protocol surface
• route unified exec and executor-backed filesystem tools through exec-server
• add remote workspace remap and executor-aware path handling
• load project docs and repo skills through the executor-backed environment
• route app-server filesystem and command/exec through the same executor seam
• add remote-path, app-server, and exec-server coverage across the stack

Review shape

This branch is the aggregate of the stacked draft PRs that build the exec-server integration incrementally:

• Harden exec-server follow-up and workspace remap #15019
• Load remote repo skills through session environment #15033
• Inject executor-aware filesystem into app-server #15034
• Route app-server command/exec through executor backend #15040
• Route remote project docs and repo skills through executor environment #15066

Notes

• base: main
• this is the single end-to-end review branch for the full exec-server stack

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