Reconcile .codex/AGENTS.md with .claude/CLAUDE.md (shared origin, drifted)

Both agent-instruction files are now tracked in this repo:

• .claude/CLAUDE.md (~133 lines) — Claude Code's global instructions
• .codex/AGENTS.md (~101 lines) — Codex's equivalent

They share a common origin (the same "experienced, pragmatic software engineer" engineering-principles doc) but have drifted apart. A rough content diff shows ~42 lines present in CLAUDE.md but not AGENTS.md (rules added to CLAUDE.md over time — e.g. the parallel-agents/file-state-race guidance, the time-estimate and trivial-work sections, expanded testing rules) and ~18 lines unique to AGENTS.md (e.g. its more specific "don't test rendered scripts" testing guidance).

This is a deliberate "look at it later" item — both were tracked as-is during the config sweep rather than reconciled in-flight.

Decision to make

• Unify: pick one canonical source and generate/symlink the other (with a tool-specific header), so a rule added once lands in both. Risk: the two tools genuinely want some different guidance.
• Keep separate, intentionally: accept per-tool divergence but do a one-time merge pass so neither is missing a rule the other has, then let them drift on purpose.

Suggested first step

Three-way against the common ancestor (if recoverable from history) to see which differences are intentional per-tool vs just stale, then fold the stale-only-in-one deltas into both.

Filed during the $HOME config-tracking sweep.

Compare .codex/AGENTS.md and .claude/CLAUDE.md, starting with their shared history and a three-way diff against the common ancestor if it can be recovered. Identify which differences are intentional per-tool guidance and which are stale; done means the chosen relationship is documented and both files reflect the agreed reconciliation.