Extract two pure clusters from the cursor executor into sibling leaves:
- cursor/prompt.ts: isRecordLike + toolChoiceDirectiveLine + buildCursorOutputConstraints
- cursor/composer.ts: composer thinking-as-content decoding (isComposerModel,
visibleComposerContentFromThinking, composerReasoningRemainder + markers)
Host imports both back for internal use and re-exports the 3 composer helpers for
external importers (tests). Host 1576 -> 1451 LOC. Byte-identical bodies (verbatim
multiset prompt 65/65, composer 32/32), leaves have zero imports (no cycle). Adds a
split-guard; consumer tests stay green (cursor-composer-thinking, cursor-streaming,
cursor-agent-tool-calls, translator-openai-to-cursor, cursor-agent-system-prompt).
