fix: make CLI default title an opt-in flag

[MbappeWU]

The CLI currently adds a fallback link title when --default-title is absent, then disables that behavior when the flag is supplied. Its argparse action is store_false, which reverses the option's documented opt-in meaning.

Use store_true so the default is false and the flag enables href-based titles. Preserve explicit titles, stdin and named-file conversion, and the existing autolinks behavior. Add tests against the public CLI entry function rather than argparse internals. No converter/API, dependency, version, config or source workflow changes.

Validation:

• Original Python 3.8.20 tox flow: 86 tests pass, flake8 passes, and README reStructuredText lint passes. The unchanged baseline has 83 passing tests. mypy . passes for 14 source files; mypy --strict tests/types.py passes.
• The exact final CLI test file against the original implementation has 2 failures and 1 pass: default/fallback and autolink assertions expose the reversed behavior; explicit-title preservation already works.
• Original application workflow in the fork succeeds in both build and types jobs using actual Python 3.8.18. It runs the unchanged tox test/lint/README flow, wheel/sdist build, mypy and strict types check. Only a manual-dispatch event was added on the separate fork CI branch; the source and publishing workflows are unchanged.
• Real wheel/sdist each contain 4 runtime files, including py.typed, matching final Git. Archive integrity and Twine checks pass. Independent replay of the retained helper plus a semantic checker verifies actual installed console/module stdout, stderr and exit for 7 cases: default/flag, stdin/file, explicit title, autolinks alone and autolinks with fallback titles. The installed environment uses Python 3.8.20 and the declared dependencies.
• Independent code and input reviews approve. The current complete open-PR diff refresh found no active fix for this option. Upstream workflow execution and maintainer review remain pending approval; no release or package upload was triggered.