Add environment registry support

[starr-openai]

Summary

• turn EnvironmentManager into an ID-keyed environment registry with default-environment fallback
• thread environment_id through thread start and public app-server fs APIs, plus add app-server environment register/list APIs
• add unit and end-to-end coverage for default, named local, and named remote environment flows

Validation

• bazel test //codex-rs/exec-server:all //codex-rs/app-server:all passes on dev
• bazel test //codex-rs/core:core-all-test --test_env=CODEX_JS_REPL_NODE_PATH=/home/dev-user/.nvm/versions/node/v25.9.0/bin/node still has 3 failing suite::tool_parallelism::* tests on dev

[starr-openai]

Replacing this single PR with the stacked environment-registry PR series: #18013 -> #18014 -> #18015 -> #18012.