fix: align fallback prompts with conservative validation

[rlippmann]

Summary

• align the README and fallback prompt templates with the conservative 0.1.x source-aware validation boundary
• remove examples that teach broader semantic rewrites which the current validator intentionally rejects
• clarify the fallback validation docstring so it accurately describes the current strict whole-message canonical matching behavior
• resolve the contract mismatch identified in fix: Source-aware fallback validation rejects canonical directive outputs for documented natural-language cases #15

Notes

• this does not broaden validator behavior or change the existing 0.1.x runtime acceptance boundary
• broader semantic drafting is deferred to the future suggest_directive design rather than being introduced as silent mutation-capable preprocessing
• uv run pre-commit run --all-files passes
• uv run pytest passes with 117 passed

Closes #15

[rlippmann]

Investigation found that the immediate failure is real, but the correct fix needs further design.

The current source_input validation added during fallback-boundary hardening requires near-canonical source/output equivalence. That prevents intended fallback drafting such as please use docker → use docker.

Attempts to relax that requirement with lexical operand grounding introduced unacceptable semantic-laundering false positives, while adding positive or negative natural-language rewrite rules would effectively create a second parser.

The proposed prompt/documentation narrowing was therefore abandoned rather than redefining intended fallback behavior to match the current validator.

Leaving this issue open pending a principled source-aware validation boundary. Broader suggest_directive work may inform the eventual design, but please use docker → use docker remains an intended fallback use case.