feat: 4.1.0 pr-review improvements (#37)

## 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.

Author: thedoublejay

crates/gather-step-cli/src/commands/mod.rs

@@ -396,6 +396,26 @@ mod tests {
         }
     }
 
+    #[test]
+    fn rejects_zero_pr_review_parallelism_during_parse() {
+        let error = Cli::try_parse_from([
+            "gather-step",
+            "pr-review",
+            "--pr-set",
+            "examples/pr-set/cross-repo-feature.yaml",
+            "--parallelism",
+            "0",
+        ])
+        .expect_err("zero parallelism should be rejected by clap");
+
+        assert!(
+            error
+                .to_string()
+                .contains("--parallelism must be an integer greater than or equal to 1"),
+            "unexpected error for zero parallelism: {error}"
+        );
+    }
+
     #[test]
     fn parses_top_level_serve_args() {
         let cli = Cli::parse_from([