Make package naming match the documented new-name cutover #30

During a correctness pass with Astra we found the following issue:

Author's preface above. Issue details below written by Astra (AI assistant) on 2026-10-09.

The README says we no longer build ros-<distro>-<package> names, but shared configuration still sets package_name_mode: both.

I confirmed this with actual Jazzy recipe generation: it creates both ros2-udp-msgs and a ros-jazzy-udp-msgs compatibility recipe. The canonical recipe has a Jazzy-qualified mutex dependency, while the compatibility recipe only depends on ros2-udp-msgs ==0.0.5.

[INFERENCE] With multiple distro channels enabled, an old distro-qualified alias can resolve an equal-version canonical package from another distribution. The audit inspected the recipes; it did not exercise a mixed-channel installation.

Code: shared/vinca.yaml, README.md and the shared-default comment in tools/maintenance.py.

Suggested change

Use package_name_mode: new, matching the documented cutover. If compatibility packages are intentional, make that an explicit documented policy and add the originating distro's mutex constraint to each alias instead.

Acceptance criteria

• Generated artifacts match the chosen naming policy and the README.
• For a new-only cutover, generation produces no legacy compatibility recipes.
• If aliases remain, a solver cannot satisfy a Jazzy alias with another distro's canonical package.
• New-distro/import behavior and comments use the same naming policy.