Add execution-environment-contract Chain Skill

[Dimon94]

Parent

#40

What to build

Add a Chain Skill that defines parallel execution graph and integration semantics. It should cover environment types, dependency and parallel eligibility fields, child dispatch boundaries, child final reports, integration gates, and worktree closeout rules.

This slice should not wrap Codex thread tools, heartbeat implementation, model selection, or platform-specific automation. Platform orchestration remains in its current adapter/reference layer.

Acceptance criteria

• Execution Environment Contract defines implementation, review, check, act, and diagnosis environment types.
• It defines dependency, parallelism, assigned task, touched path, mutable resource, and verification fields.
• It defines required child dispatch boundary content and child final report shape.
• It defines integration gates including clean worktree, commit proof, touched-path audit, focused verification, and cherry-pick ownership.
• It defines safe worktree closeout rules.
• It does not own Codex thread tool calls, heartbeat implementation, model selection, or platform UI behavior.
• Focused tests or contract checks prove parallel orchestration references the contract.

Blocked by

• Add task-contract Chain Skill #42
• Add workflow-chain-contract Chain Skill #43