Add more URL constants

[rolandwalker]

Description

Motivation: reduce typos, reduce search/replace.

Incidentally, capitalize lowercase "issue" when referring to GitHub.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Changelog regression (duplicate release heading)
   The PR introduces a second 1.58.0 (2026/02/28) section at the top, which duplicates the existing release block and fragments entries. This should be merged into the existing 1.58.0 section instead of creating a new one.
   Reference: changelog.md:1, changelog.md:9

2. Test gap for modified user-facing warning/help text
   Several warning/help strings were changed to use constants (HOME_URL, ISSUES_URL, REPO_URL), but there are no matching test updates to lock output format/URLs. Add assertions in CLI output tests (likely in test_main.py) for at least one --ssl deprecation warning and one --checkup/support-text path to prevent accidental regressions.
   Reference: main.py:1953, main.py:2215, main.py:2583, test_main.py

No direct security issues found in this diff.

[scottnemes]

Don't think issue should be capitalized, but fine as-is if you prefer.