chore: TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 0
Acknowledges Rule #0 of the AI Agent Checklist (lines 809-940 of the styleguide). Sections re-read for this track: - 5 Patterns (Nil-Sentinel, Zero-Init, Fail-Early, AND over OR, Error Info as Side-Channel) - Drain Points (5 patterns + 5 'NOT a drain point' anti-patterns) - Boundary Types (third-party SDK, stdlib I/O, FastAPI) - Broad-Except Distinction (the table classifying every catch site by what it does with the exception) - AI Agent Checklist (5 MUST-DO + 7 MUST-NOT-DO + 3 boundary patterns) Key principle applied to this track: 'error dropping is NOT a drain' (the legacy wrapper def _x(): return _x_result(...).data defeats the entire purpose of the Result[T] migration; the wrapper silently swallows the error from _x_result).
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