Prepare ReproMin v0.1.0.dev12

[fly1d]

Outcome:
Prepare the next GitHub prerelease and remove the broken handoff after the advertised one-off uvx demo.

Context and impact:
The public dev11 build is six commits behind main. A clean first-run audit also showed that uvx completes the demo, then its temporary environment disappears, so the printed bare repomin validation and Doctor commands fail with exit 127. The demo now states that persistent installation is required and links directly to the Quick Start and support routes.

Changes:

• Bump the runtime and all current install/Action references to 0.1.0.dev12.
• Move the accumulated user-facing changes into the dated dev12 changelog section.
• Remove unusable post-uvx demo commands and cover the handoff with a regression assertion.

Validation:

• Contributor preflight: 766 tests passed, 4 platform skips; docs, Ruff, and byte compilation passed.
• Offline benchmarks: 16 passed, 0 skipped, 0 failed.
• Wheel and source distribution: built, Twine-checked, structurally validated, installed separately, Quick Start passed, full installed suites passed, and pip check passed.
• git diff --check passed.

Risk:
Low. The runtime change affects only demo guidance; reduction behavior and public Action inputs are unchanged. PyPI publishing remains disabled.

Done when:
Required CI is green and the reviewed commit is ready for the immutable v0.1.0.dev12 release-candidate tag.

Disclosure:
Codex performed the first-run audit, implementation, tests, and PR drafting under the repository owners direction. No independent human code review has been recorded.