[exec-server] Add MVP startup and surface seams

[starr-openai]

Summary

• add typed thread-start bundle startup payloads and session-contract plumbing for cwd/approval/sandbox handling
• add materialized bundle seams across external agent config, project-doc discovery, and skills loading
• add prompt/tool surface inventory scaffolding so exec-server-relevant startup surfaces are represented explicitly

Validation

• cargo check -p codex-app-server -p codex-app-server-protocol -p codex-core -p codex-core-skills -p codex-exec -p codex-tools
• cargo run -p codex-cli -- --help
• cargo run -p codex-cli -- app-server --help

Notes

• this is an MVP seam-setting slice, not a complete docker-backed e2e proof
• remote devbox validation is still blocked by local SSH-agent signing failure (agent refused operation)

[starr-openai]

Superseded by the narrower exec-server split follow-ons, especially #16736, #17030, and #16937.