feat: 4.1.0 pr-review improvements

[thedoublejay]

Summary

Prepares the v4.1.0 pr-review improvements:

• adds PR-set manifest validation and neutral examples
• adds coordinated multi-PR review execution with dependency ordering, partial failure reporting, and cross-PR contract drift
• adds GitHub search based PR-set generation and direct --from-gh execution
• supports reviewing child repos with a parent workspace config
• exposes the new PR-set and config/cache controls in CLI help, MCP schemas, and docs
• adds the v4.1.0 docs changelog entry

Context / Motivation

pr-review previously handled one base/head pair at a time. Related PRs, stacks, and cross-repo feature sets needed a manifest-level workflow so the review can preserve dependency order and report set-level contract drift.

The MCP wrapper also needed to expose the same controls as the CLI so assistant consumers can discover parent config, cache root, baseline-check, GitHub-query, set id, and parallelism options from the tool schema.

Key Decisions

• Keep PR-set examples neutral and reusable.
• Require exactly one PR-set source for MCP pr_review_set: pr_set or from_gh.
• Resolve MCP path inputs relative to the configured workspace root.
• Keep v4.1.0 marked as unreleased in the docs changelog; version stamps are not bumped in this branch.

Verification

• cargo test -p gather-step-mcp pr_review -- --nocapture
• cargo test -p gather-step pr_review::multi_pr:: -- --nocapture
• cargo clippy -p gather-step -p gather-step-mcp --all-targets -- -D warnings
• target/debug/gather-step pr-review --help
• npm run build in website
• git diff --check
• changed-line scan for workspace-specific terms

Follow-ups

• Mark v4.1.0 as released and bump version stamps when cutting the release.

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