refactor(skill): streamline zest-dev for GPT-6 Astra

[nettee]

Summary

• reduce duplicated and overly prescriptive guidance across the Zest Dev router and section guides
• preserve lifecycle, CLI, design-record, EAG, DFU, ticket, and deviation contracts
• retain the to-tickets reference as optional guidance and calibrate implementation verification for Astra
• loosen deployment tests from incidental wording while keeping stable workflow invariants covered

Scope

Subagent definitions and command prompts are unchanged.

Verification

• skill-creator quick validator
• pnpm test:local (23 passed, 1 skipped)
• git diff --check