The prompt surface only emitted the full checklist or full command block, so agents had to paste extra guidance or duplicate those lines into docs. This adds named prompt parts, list output, and exec-mode validation so callers can fetch only the section they need without changing the default outputs.

Constraint: Existing gx prompt, gx prompt --exec, and deprecated prompt aliases had to stay compatible
Rejected: Trim AGENTS.md again instead | user asked to route the prompt slices through gx itself
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Add future agent guidance as named gx prompt parts before copying it into repo docs or templates
Tested: node --check src/context.js
Tested: node --check src/cli/main.js
Tested: node --test test/prompt.test.js
Tested: openspec validate agent-codex-improve-gx-prompt-parts-for-token-usage-2026-04-22-16-05 --type change --strict
Tested: openspec validate --specs
Not-tested: gx branch finish network/PR path before running the finish pipeline