[runtime] introduce runtime/install api

[acrognale-oai]

Summary

• Add the v2 runtime/install request and response surface and route app-server requests through the selected execution environment.
• Install runtime artifacts end to end: download, checksum verification, archive extraction, runtime metadata handling, and validation of required Node, node_modules, and Python paths.
• Finalize successful installs by publishing bundled skills and plugin marketplaces into the target environment's CODEX_HOME, clearing app-server caches so installed resources are visible.
• Make publication rollback-safe through staged replacement and backup restoration, and reject invalid legacy skill removal paths before modifying disk state.

Test Plan

• Regenerated app-server schema fixtures; formatting and scoped Clippy checks passed.
• codex-app-server-protocol, codex-exec-server, and focused codex-app-server runtime-install tests passed.

Stack

1. #24111 - Introduce runtime/install API
2. #24112 - Add progress and cancellation APIs
3. #24113 - Forward runtime APIs from app-server to exec-server