[P2-018] Align repository instructions with the delivery model

[deku2026]

Summary

Aligns this repository with the parallel delivery model of P2-018: implementation is scheduled as delivery tasks rather than one numbered Current task. Documentation only. No source, dependency, package, workflow or policy-data change, and no pending product work is started.

• AGENTS.md gains a "Delivery model (P2-018)" section:

  • the accepted baseline and where this repository's tasks live;
  • the claim and adoption-slice entry rules;
  • concurrent workers in separate worktrees inside task write scopes;
  • the declared shared-file protocols for this repository;
  • task-ID pull request titles and claim-epoch merges.

  The P2-017 validation rules are unchanged.

• AGENTS.md: the PR-title rule no longer refers to "the current work package/substep".

Validation

• Existing file edited only; the provenance inventory is unchanged. Repository policy checks run in the configured CI (no local build for a documentation change).
• No product build or runtime test (documentation-only change); the repository's configured CI remains required before merge.

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