* Only run snapshot-bump workflow when `.version` changed

The `snapshot-bump` workflow used to run on every non-scheduled, non-`bump`
commit on `main`. Combined with `create_next_snapshot_version`'s only
de-duplication being "is the bump branch currently on the remote?", any
commit that landed on `main` between a release and the merge of its
follow-up bump PR could open a duplicate bump PR (e.g. #6720 was opened
20s after #6719 merged because `bump/<v>` had just been deleted).

Compute `version_file_changed` in the dynamic-config setup job by diffing
the triggering commit against its parent for `.version`, forward it via
`continuation/continue`'s `parameters` arg, and add it as a `when`
condition on the `snapshot-bump` workflow. Now the workflow only triggers
on commits that actually modified `.version` — i.e. the release commit
itself (where the lane runs the bump) and the bump merge commit (where
the lane no-ops because `.version` is now a SNAPSHOT). Unrelated commits
on `main` no longer race against an in-flight bump PR.

Made-with: Cursor

* Move continuation-parameter computation to a Node script

Mirrors the existing `.circleci/generate-requested-jobs-config.js` pattern
so future derived pipeline parameters can be added without growing the
inline shell in `.circleci/config.yml`.

Made-with: Cursor

* Drop now-redundant comment above continuation-parameters step

The Node script's name and its own comments already convey what this
step does.

Made-with: Cursor

* Drop redundant comments in default_config.yml

The parameter name is self-explanatory and the rationale already lives
in the generate-continuation-parameters.js script and commit history.

Made-with: Cursor