in init setup- offer to add repo key to Cargo.toml for those missing it

the number one error i see with end users is that they realize they are missing the repo key in their Cargo.toml or that it is incorrect. in general, cargo-dist has the info it needs to update this (as it is almost certainly running in a local git checkout) - for situations where we can safely and confidently offer to update Cargo.toml- i think we should do so in init

current:
```
✔ enable Github CI and Releases? · yes

× Github CI support requires you to specify the URL of your repository
help: Set the repository = "https://github.com/..." key in your Cargo.toml
```

proposal:
```
✔ enable Github CI and Releases? · yes

? You haven't specified a repository key in your Cargo.toml Set it to ? · yes
```

Start in the init flow and inspect how cargo-dist reads and updates Cargo.toml, then trace the local Git checkout information available for deriving the repository URL. Define the safety checks and prompt behavior described in the proposal, including the missing or incorrect repository-key path; the issue names no files or tests.

Written by the indexing model from the issue text.