feat(config-migration): preserve comments when migrating renovate.json

[felipecrs]

Changes

This PR leverages the jsonc-morph library to preserve comments and formatting when migrating renovate.json configuration files.

In my opinion this is the last piece missing to make renovate.json the go-to configuration file format for Renovate. Perhaps deprecation of renovate.json5 can be even considered in the future.

Context

Please select one of the below:

• This closes an existing Issue: Replace JSON config migration code with jsonc-parser #27972
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Note

I used Claude Sonnet 4.5 to kickstart json-morph magic, and then used it to generate the tests.

However, I carefully reviewed and made substantial modifications to the generated code afterwards.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[felipecrs]

This was a lot more complicated than I thought. I can probably cleanup the logic a little more, but it's already too late here.

I'm marking this as ready to review anyway to maybe collect some feedback.

[felipecrs]

I'll probably create a library out of this, and name it jsonc-patch or something like that.

This is generic and useful enough. The very few Renovate-specific things can be parameterized.

Would keep the code in Renovate tidy as well.

Please let me know if you have any objection with that.

[jamietanna]

IMO, that sounds reasonable, @felipecrs!

One thing to be cautious around is whether you'll be taking the net-new code into a new library, or if there's any code from inside Renovate that will need to go alongside it into the library.

Given Renovate is AGPL-3.0-only licensed, that'll have implications for the license on the library

Happy to help work out what makes sense here!

[felipecrs]

Nothing from Renovate itself would compose the library, so no need to worry about that. But thanks for the hint! :D

[jamietanna]

You're welcome!

OK, in which case, wanna put this into a Draft PR state, so you can split the library out + then bring it back in here when that's ready?