feat: add milestone management workflow

[hussain-s]

Add milestone management automation for release tracking.

Changes

• add .github/workflows/milestone-management.yml to auto-assign milestones on merged PRs
• support merged PRs targeting main and release/test branch patterns
• add workflow_dispatch support to create or re-open upcoming X.Y.Z milestones
• document milestone naming and branch mapping in scripts/README.md
• add scripts/milestone_management_preview.py for local milestone-selection checks
• add unit coverage for the branch-to-milestone logic in tests/test_milestone_management_preview.py

How I tested this

• ran .venv311/bin/python -m pytest tests/
• ran PRE_COMMIT_HOME=.pre-commit-cache .venv311/bin/pre-commit run --all-files
• ran .venv311/bin/python -m pytest tests/test_milestone_management_preview.py
• exercised scripts/milestone_management_preview.py manually for main, release branches, and unmanaged branches

Notes

• milestone titles follow the X.Y.Z convention requested in issue ci: milestone management workflow #722
• merged PRs to main pick the earliest open X.Y.Z milestone
• merged PRs to release/test branches use the milestone inferred from the branch name
• milestone creation is manual through workflow_dispatch; merged-PR assignment is automatic once milestones exist

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[andreahlert]

burr supports py3.9 per pyproject.toml but str | None on line 42 needs 3.10+. import breaks on 3.9 and the test file blows up with it since it loads the module at top level.

Suggested change

	import argparse
	import re
	from __future__ import annotations
	import argparse
	import re
	import sys

[andreahlert]

this fails the workflow run on every merge until a milestone exists. a console.warn + return would skip quietly instead of spamming failed-run notifications.

Suggested change

	if (!milestone) {
	throw new Error(
	"No open X.Y.Z milestone found for main. Create one with the workflow_dispatch path first."
	console.warn(`#${pr.number}: no open X.Y.Z milestone for main, skipping assignment.`);
	return;

[andreahlert]

the JS in the workflow has the same logic but no tests. preview script mirrors it today, will drift later. worth testing the JS too, or at least a comment in both files saying they must stay in sync.

[andreahlert]

missing coverage for release/0.43.1 (slash variant), apache-burr-0.43.1-release, and the release-0.43 minor fallback. regex handles them but nothing exercises that path.