Add AGENTS and codex project conventions

codex/README.md

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+# Codex Project Notes
+
+This directory stores project-specific Codex working conventions.
+
+Use these files as the startup layer for future sessions in `pbf`.
+
+## Files
+
+- [`SESSION_START.md`](/root/sourceserver/pbf/codex/SESSION_START.md)
+  - what to read first when resuming work
+- [`RESUME_PROMPT.md`](/root/sourceserver/pbf/codex/RESUME_PROMPT.md)
+  - a ready-to-use prompt for continuing the project
+
+## Core Rule
+
+If there has been meaningful progress, update:
+
+- [`STEP_RECORD.md`](/root/sourceserver/pbf/STEP_RECORD.md)
+
+That file is the current source of truth for "where we stopped last time."