docs: make Copilot primary executor in agent instructions

[Copilot]

Codex issue comments are not verifiable GitHub artifacts; only a PR, branch, commit, or merge constitutes delivery. These docs lacked an explicit executor policy to encode that distinction.

Changes

AGENTS.md

• New ## Executor policy section declaring Copilot as primary executor for bounded issue-first PR work
• Codex scoped to reviewer / cross-repo analyst / backup — explicitly not the default executor
• Delivery definition: GitHub must show a PR, branch, commit, or merge; comments alone do not count
• Issue body called out as the sole task spec (Copilot may not see post-assignment comments)

.github/copilot-instructions.md

• Matching ## Executor policy section prepended, reinforcing the same delivery and source-of-truth rules
• Codex comment caveat surfaced at the point Copilot reads its own instructions

All existing high-risk path cautions preserved verbatim in both files.

[mdheller]

Replay review:

This executor-policy update is strategically correct, but the PR is non-mergeable against current main; do not merge stale history.

Preserve the useful policy:

• Copilot is the primary executor for bounded issue-first PR work;
• Codex is reviewer / cross-repo analyst / backup, not default executor;
• issue comments alone are not delivery artifacts;
• the issue body must contain the full task spec because Copilot may not see later comments.

Recommended action: replay this policy into current AGENTS.md and .github/copilot-instructions.md, then align the wording with the AgentPlane Agentic PR Control Plane replay. Close this PR as superseded after replacement lands.