Stop writing default dependentChangeType in change files

[Elizabeth Craig (ecraig12345)]

If no custom value is specified for dependentChangeType when generating change files, omit the default value from the written file, and instead fill it in during bumping.

This is related to #947 and opens up options for some potential future improvements (plus reducing the amount written to many small files in a large repo isn't a bad thing).

[Copilot]

Pull request overview

Moves default dependentChangeType handling from change-file generation to bump calculation while preserving explicit overrides.

Changes:

• Omits default dependent bump values from generated change files.
• Applies none or patch defaults during bumping.
• Updates validation, tests, documentation, fixtures, and change-file guidance.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
skills/beachball-change-file/SKILL.md	Updates change-file generation guidance.
packages/beachball/src/validation/validate.ts	Allows omitted dependent change types.
packages/beachball/src/types/ChangeInfo.ts	Makes the field optional and documents defaults.
packages/beachball/src/changefile/promptForChange.ts	Writes only explicit overrides.
packages/beachball/src/bump/updateRelatedChangeType.ts	Applies defaults during bumping.
packages/beachball/src/__tests__/changefile/promptForChange_getChangeFileInfoFromResponse.test.ts	Tests field omission.
packages/beachball/src/__tests__/bump/updateRelatedChangeType.test.ts	Tests bump-time defaults.
packages/beachball/src/__functional__/changefile/readChangeFiles.test.ts	Updates expected change-file contents.
packages/beachball/src/__fixtures__/changeFiles.ts	Removes the fixture’s explicit default.
docs/overview/v3-migration.md	Documents the v3 behavior change.
change/beachball-cddc125a-9fbf-42ac-9343-261e46698091.json	Records the Beachball breaking change.
change/@microsoft-beachball-change-file-skill-ba33783c-4380-41f5-baf6-4c49507c2837.json	Records the skill update.

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