Require user decisions before relaxing flow goals

[nzy1997]

The autonomous flow workflow could relax a hard goal during a pivot and still report the result as solved. This PR keeps the original goal and test authoritative: a materially easier target becomes a proposal for the user, and accepted changes are recorded as a new goal rather than silently weakening success criteria.

The change follows OpenAI's Rethinking skills and prompts for GPT-6 Astra, especially explicit authorization boundaries and honest completion states. It is the flow slice split from #62 after review feedback.

Validation

• python3 scripts/validate_skills.py: 16 skills pass
• targeted repository tests: 56 passed
• python3 -m pytest -q: 268 passed, 5 skipped
• git diff --check: clean

[GiggleLiu]

Closing. how-to-flow is fully autonomous by design; pausing a pivot for a user decision while "continuing work on the original goal" is not coherent inside an unattended loop, and relaxing the preflight fact-loading gate removes what makes the loop CDCL-like. The concern (drift reported as solved) is already covered by the final check on main, which requires the original goal, not a drifted one, and PIVOTED-SOLVED already reports the gap to the original.