chore: unify versioning

[artus9033]

Summary

This PR introduced the following:

• Unified all package versions to 3.0.0-rc.2 across brownfield, brownie, cli, and react-native-brownfield
• Enabled synchronized (fixed) versioning for all packages in changesets config ("fixed": [["*"]])
• Normalized repository.url fields in package.json files to use the git+...git format
• Updated docs compatibility table to include version 3.x for RN 0.81.x/0.82.x
• Added cross-package compatibility note in docs guaranteeing same-major-version interop for >= 3.0.0
• Enforced synchronized versioning with the fixed strategy in changesets

Test plan

CI green