Add short hostname prompt format string: \H

[rolandwalker]

Description

The \h and \H feel backwards, but \h already existed, and reversing them would be changing the defaults.

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. Missing regression tests for new \H token
   \\H was added in prompt rendering, but there’s no test coverage in test/test_main.py for the new behavior (only existing \\h cases are covered). This leaves edge behavior unverified and easy to break in future refactors.

• Code path: mycli/main.py#L1570
• Existing nearby prompt tests: test/test_main.py#L329

Action: add focused tests for:

• FQDN host (db01.prod.example.com) -> \\H renders db01
• no host (host=None) -> \\H renders localhost
• optional: login_path_as_host=True path to confirm intended \\H behavior with login aliases

No correctness/security regressions were obvious from the diff beyond this test gap.

[scottnemes]

Verified short hostname value works to show the shortened hostname