docs(changelog): record byte5ai engineering-standards bootstrap

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Summary

• Backfills the CHANGELOG entry that should have shipped with PR chore: apply byte5ai engineering-standards (initial bootstrap) #31 (byte5ai engineering-standards bootstrap).
• Per AGENTS.md the standards rollout (.github/engineering-standards.yml, .hooks/pre-push, script/setup, AGENTS/CONTRIBUTING updates, branch protection wiring) qualifies as an ops change and required a same-step CHANGELOG note.
• Doubles as the first end-to-end CI verification after GitHub Actions were reactivated on 2026-05-17 (Actions had been disabled since 2026-05-11).

Test plan

• All five required status checks reach a terminal state:
  • middleware (lint + typecheck + test)
  • web-ui (lint + typecheck + vitest)
  • schema (migrations on pgvector)
  • audit (high+critical block) (middleware)
  • audit (high+critical block) (web-ui)
• CHANGELOG renders correctly on GitHub.

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Superseded by #35 (consolidated CI resurrection). Under the strict branch protection with strict: true required checks, the three PRs (#32/#33/#34) were mutually blocking — each had at least one red required check depending on one of the others. All changes from this PR are preserved in #35 (cherry-picked under the same author identity), plus a fourth commit that fixes the prefer-const lint bug that had been hidden behind EBADENGINE, and an extended CHANGELOG ### Fixed block.