feat: add script for generating release notes (#6519)

This PR adds a script to automatically generate release notes using the
new `changelog-*` labels and "This PR ..." conventions.

Usage:
```
script/release_notes.py v4.X.0
```
where `v4.X.0` is the **previous** release, i.e. the script will process
all commits *since* that tag.

script/release_notes.py

@@ -0,0 +1,145 @@
+#!/usr/bin/env python3
+
+import sys
+import re
+import json
+import requests
+import subprocess
+from collections import defaultdict
+from git import Repo
+
+def get_commits_since_tag(repo, tag):
+    try:
+        tag_commit = repo.commit(tag)
+        commits = list(repo.iter_commits(f"{tag_commit.hexsha}..HEAD"))
+        return [
+            (commit.hexsha, commit.message.splitlines()[0], commit.message)
+            for commit in commits
+        ]
+    except Exception as e:
+        sys.stderr.write(f"Error retrieving commits: {e}\n")
+        sys.exit(1)
+
+def check_pr_number(first_line):
+    match = re.search(r"\(\#(\d+)\)$", first_line)
+    if match:
+        return int(match.group(1))
+    return None
+
+def fetch_pr_labels(pr_number):
+    try:
+        # Use gh CLI to fetch PR details
+        result = subprocess.run([
+            "gh", "api", f"repos/leanprover/lean4/pulls/{pr_number}"
+        ], capture_output=True, text=True, check=True)
+        pr_data = result.stdout
+        pr_json = json.loads(pr_data)
+        return [label["name"] for label in pr_json.get("labels", [])]
+    except subprocess.CalledProcessError as e:
+        sys.stderr.write(f"Failed to fetch PR #{pr_number} using gh: {e.stderr}\n")
+        return []
+
+def format_section_title(label):
+    title = label.replace("changelog-", "").capitalize()
+    if title == "Doc":
+        return "Documentation"
+    elif title == "Pp":
+        return "Pretty Printing"
+    return title
+
+def sort_sections_order():
+    return [
+        "Language",
+        "Library",
+        "Compiler",
+        "Pretty Printing",
+        "Documentation",
+        "Server",
+        "Lake",
+        "Other",
+        "Uncategorised"
+    ]
+
+def format_markdown_description(pr_number, description):
+    link = f"[#{pr_number}](https://github.com/leanprover/lean4/pull/{pr_number})"
+    return f"{link} {description}"
+
+def main():
+    if len(sys.argv) != 2:
+        sys.stderr.write("Usage: script.py <git-tag>\n")
+        sys.exit(1)
+
+    tag = sys.argv[1]
+    try:
+        repo = Repo(".")
+    except Exception as e:
+        sys.stderr.write(f"Error opening Git repository: {e}\n")
+        sys.exit(1)
+
+    commits = get_commits_since_tag(repo, tag)
+
+    sys.stderr.write(f"Found {len(commits)} commits since tag {tag}:\n")
+    for commit_hash, first_line, _ in commits:
+        sys.stderr.write(f"- {commit_hash}: {first_line}\n")
+
+    changelog = defaultdict(list)
+
+    for commit_hash, first_line, full_message in commits:
+        # Skip commits with the specific first lines
+        if first_line == "chore: update stage0" or first_line.startswith("chore: CI: bump "):
+            continue
+
+        pr_number = check_pr_number(first_line)
+
+        if not pr_number:
+            sys.stderr.write(f"No PR number found in {first_line}\n")
+            continue
+
+        # Remove the first line from the full_message for further processing
+        body = full_message[len(first_line):].strip()
+
+        paragraphs = body.split('\n\n')
+        second_paragraph = paragraphs[0] if len(paragraphs) > 0 else ""
+
+        labels = fetch_pr_labels(pr_number)
+
+        # Skip entries with the "changelog-no" label
+        if "changelog-no" in labels:
+            continue
+
+        report_errors = first_line.startswith("feat:") or first_line.startswith("fix:")
+
+        if not second_paragraph.startswith("This PR "):
+            if report_errors:
+                sys.stderr.write(f"No PR description found in commit:\n{commit_hash}\n{first_line}\n{body}\n\n")
+                fallback_description = re.sub(r":$", "", first_line.split(" ", 1)[1]).rsplit(" (#", 1)[0]
+                markdown_description = format_markdown_description(pr_number, fallback_description)
+            else:
+                continue
+        else:
+            markdown_description = format_markdown_description(pr_number, second_paragraph.replace("This PR ", ""))
+
+        changelog_labels = [label for label in labels if label.startswith("changelog-")]
+        if len(changelog_labels) > 1:
+            sys.stderr.write(f"Warning: Multiple changelog-* labels found for PR #{pr_number}: {changelog_labels}\n")
+
+        if not changelog_labels:
+            if report_errors:
+                sys.stderr.write(f"Warning: No changelog-* label found for PR #{pr_number}\n")
+            else:
+                continue
+
+        for label in changelog_labels:
+            changelog[label].append((pr_number, markdown_description))
+
+    section_order = sort_sections_order()
+    sorted_changelog = sorted(changelog.items(), key=lambda item: section_order.index(format_section_title(item[0])) if format_section_title(item[0]) in section_order else len(section_order))
+
+    for label, entries in sorted_changelog:
+        section_title = format_section_title(label) if label != "Uncategorised" else "Uncategorised"
+        print(f"## {section_title}\n")
+        for _, entry in sorted(entries, key=lambda x: x[0]):
+            print(f"* {entry}\n")
+
+if __name__ == "__main__":
+    main()