Add task-contract Chain Skill

[Dimon94]

Parent

#40

What to build

Add the Task Contract as a pure reference Chain Skill that defines the durable meaning of task.md for planning, execution, verification, review, and closeout. Core PDCA skills should read it instead of each carrying isolated task contract prose.

This slice should keep the contract focused on task.md shape and semantics. It must not introduce a parser, validator, CLI command, workflow router, or delivery policy.

Acceptance criteria

• Task Contract is implemented as a Chain Skill, not a User Entry Skill.
• The contract defines task.md sections, status semantics, task block semantics, Failure Ledger semantics, Execution Environments semantics, and reader/writer responsibilities.
• Core PDCA skills read the Task Contract.
• The skill is distributed but not classified as a public user entry skill.
• Retired task-contract CLI/process artifact bans remain active.
• Focused tests cover source skill wiring and publish validation.

Blocked by

• Add skill-authoring-gate Chain Skill #41