[codex] Add line comment toggle

[cloudmanic]

Summary

• Add a repository contributor guide in AGENTS.md.
• Add a line-comment toggle for editable text tabs, available from the action menu and Esc /.
• Preserve existing indentation by placing the comment marker at the start of each line and leaving the original line content untouched after it.
• Update README hotkey docs and add editor/app coverage for toggle behavior, menu enablement, and leader dispatch.

Validation

• go test ./internal/editor ./internal/app
• make test