docs: design, plan and follow-ups for the hil_report consolidationhil-report

Records why the report code was spread across three modules and what the
consolidation buys, so the next reader does not have to re-derive it from the
diff. Names the new functions explicitly rather than presenting the change as
pure code motion -- that framing points reviewers away from the code that
carried the defects.

Three findings this PR deliberately does not close get one handoff each, per
CLAUDE.md: the worker-result tuple hil_report still unpacks positionally,
SKILL.md's no-boards rule drifting from the code, and write_report's two
non-atomic writes. Drops the pr3836 handoff, which this branch implements.

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