CI: add conservative change-based validation with a reliable required gate

Problem

Every PR update runs expensive validation even when a narrowly classified change cannot affect product behavior. Eliminate demonstrably unnecessary work while keeping required checks trustworthy.

Turborepo supports affected-package selection, but Zoo imports shared source outside package-local directories. A rule such as “run webview tests only for webview-ui changes” would miss real dependencies. Classification correctness needs validation independent of any runner-contention hypothesis.

Implementation

• Start with an explicit allowlist of documentation-only changes whose files are not build/runtime/test inputs. Treat embedded fixtures, executable examples and generated inputs conservatively.
• Classify the full relevant change range, including renamed/deleted files and mixed changes.
• Run comprehensive checks for shared code, dependencies, toolchain/workflow configuration, unknown paths, missing history or incomplete change data.
• Keep comprehensive merge-queue and release validation initially.
• Keep an always-running required gate that validates classification and expected job outcomes. Missing/failed classification must block or select full validation.
• Preserve current required check contracts or update rulesets deliberately with no validation gap.
• Validate representative historical changes and record proposed selections before enabling skips. Start narrow; expand only with dependency evidence.

Acceptance criteria

• Documentation-only allowlist, mixed changes, shared source, dependency/config changes, renames/deletions, missing base/history and classifier failures are covered.
• Full changed-file enumeration cannot silently truncate; uncertain input runs everything or blocks.
• Required gate reports for every relevant PR and merge-group event and rejects missing/failed required validation.
• Required workflows are not left permanently pending by workflow-level path filters.
• Full merge-queue/release coverage remains intact; skipped PR work is explicitly explained.
• Record classified PR categories, skipped work, runner minutes and required-check completion time; document a full-validation override and rollback.

Broader package selection should follow the shared-input audit; existing coverage slicing in #47 remains separate. A comprehensive merge-queue run is additional protection, not a substitute for correct PR classification.

Sources: Turbo CI guidance, affected selection/history requirements, GitHub required checks and skipping hazards.