[aymericbeaumet]

Summary

• Set the binary version to 2.0.0 for the incompatible configuration schema already merged on main.
• Introduce a separate @v2 reusable workflow and generated instance workflow. The existing @v1 tag continues to select v1 binaries.
• Document the 1.12.0 to 2.0 migration and fix the reusable workflow's missing cache step ID.
• Let CI test the newest published channel while a new major is being bootstrapped; after publication it tests v2.

Test plan

• cargo test --bin aggr commands::init::tests::the_shipped_snippets_are_valid passed.
• cargo fmt --all -- --check, actionlint, and git diff --check passed.
• Full local make check ran out of disk space during Rust compilation; the PR CI matrix will provide full validation.

Release plan

After this PR passes CI and merges, tag the merge commit v2.0.0. Once all six release binaries and checksums are published, create the stable v2 workflow tag and migrate the instance to @v2 with the new config keys.