Pi stage dispatches wrap dispatch-build artifacts

[clkao]

Pi stage dispatches now wrap canonical dispatch-build artifacts instead of hand-written replacement prompts.

What changed

• Added a Pi subagent stage-dispatch wrapper helper.
• Preserved dispatch-build assignment content through Pi wrapper fields.
• Added fixture smoke coverage for build artifact to worker durable state evidence.
• Locked manual fallback and no-acceptance guidance.

Evidence

• Focused dispatch, piruntime, and skill tests: passed.
• Go baseline and race suites: passed.

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