fix: abstain on compound directive input

[rlippmann]

Summary

• detect multiple canonical directive starts across the normalized whole message
• make heuristic preprocessing abstain instead of collapsing compound input into one candidate
• preserve singular directives whose payloads contain ordinary conjunctions such as and
• reuse shared canonical directive-start definitions for compound detection in both heuristic preprocessing and output validation
• add focused tests and fixtures for directive-family combinations, separators, multiline input, lexical boundaries, and singular payloads
• document that the drafter emits at most one candidate directive and does not split compound input

Notes

• detection is intentionally lexical and conservative; it does not attempt general semantic segmentation
• ordinary conjunctions are not treated as compound intent by themselves, so payloads such as use peanut butter and jelly remain valid
• context-compiler independently enforces the single-directive invariant at the authoritative engine boundary
• this change affects acquisition-layer behavior only
• validation passed:
  • uv run pre-commit run --all-files
  • uv run pytest — 111 passed

Closes #14