Disable env-bound tools when exec server is none

[starr-openai]

Summary

• make CODEX_EXEC_SERVER_URL=none a first-class disabled environment mode
• skip project-doc and hierarchical AGENTS injection when the environment is explicitly disabled
• thread environment capabilities into tool configuration so env-bound tools are omitted when exec/filesystem access is unavailable
• keep handler-level backstops so bypassed tool registrations still reject safely

Testing

• just fmt
• cargo test -p codex-exec-server environment
• cargo test -p codex-tools disabled_environment_omits_environment_backed_tools
• cargo test -p codex-tools disabled_environment_capabilities_turn_off_exec_backed_tools
• cargo test -p codex-core project_doc