feat: add self-check tip to check output

[PascalThuet]

Summary

Closes #2573.

Keeps specify check as an offline environment scan and adds a discoverability tip for the existing specify self check version lookup.

Changes

• Add a specify self check tip to specify check output.
• Keep specify check network-free; no --latest option is added.
• Document the offline/version-check split in the core command reference and upgrade troubleshooting.
• Add tests that assert the tip is shown and that specify check does not call the latest-release lookup.

Validation

Local:

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

CI:

• GitHub Actions will run the full PR validation suite.

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, adjust the PR based on review feedback, add tests/docs, and run local validation. The changes were reviewed before submission.

[mnriem]

Thanks for the work here! We left a suggestion on #2573 — specify check is an offline environment scan and specify self check already owns the version-lookup logic. Rather than adding --latest with network code, we think a simpler tip line in specify check output would solve the discoverability problem without blurring the two commands' responsibilities:

Tip: Run 'specify self check' to verify you have the latest CLI version.

Would you be open to adjusting the PR in that direction?

[PascalThuet]

Updated in e886a15 based on the suggestion: removed the --latest approach, kept specify check network-free, added the specify self check tip, and updated docs/tests. Local validation passes: targeted pytest, ruff, and diff whitespace check.