The native-harness checklist flatly marked all capabilities "required", but
even codex-native (one of the most complete native harnesses) fails several.
Reorganize the Part 2 checklist into P0 (core), P1 (parity), and Stretch
(vendor-dependent) tiers, and add capability rows surfaced by a codex-native
audit: tool-output streaming granularity, working-tree diff, generated/viewed
media, and vendor-specific modes.
Refs: #1254#1255#1256#1257#1258
Co-authored-by: Isaac
Pat Sukprasert
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