feat: add check latest option

[PascalThuet]

Summary

Closes #2573.

Adds an opt-in specify check --latest release check while keeping plain specify check offline by default.

Changes

• Add --latest to specify check.
• Extract latest-release output into a helper shared by specify self check and specify check --latest.
• Keep the existing specify self check output behavior.
• Document the new option in the core command reference and upgrade troubleshooting, including when to use it to detect local CLI drift.
• Add tests for offline default behavior, update available, up to date, unknown installed version, and lookup failures.

Validation

Local:

• uv run pytest tests/test_check_tool.py tests/test_upgrade.py -v
• uvx ruff check src/ tests/test_check_tool.py tests/test_upgrade.py
• git diff --check origin/main
• uv run specify check --help
• uv run specify self check --help

CI:

• Full pytest, ruff, and markdownlint coverage runs via GitHub Actions on the PR.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Used AI assistance to inspect the existing check/self-check flow, implement the scoped CLI option, add tests/docs, and run local validation. The changes were reviewed before submission.