fix(renovate): self-merge instead of relying on GitHub native auto-merge

[JSisques]

Summary

• Set platformAutomerge: false in the shared Renovate preset. Renovate now merges eligible PRs itself via the API once all checks pass, instead of asking GitHub's native auto-merge queue to do it.
• Root cause: GitHub's native auto-merge only works when Settings → General → Allow auto-merge is enabled on the target repo. Testing against open Renovate PRs showed this was off on gardenia-api (confirmed via the enable_pr_auto_merge API returning "Auto-merge is not enabled for this repository"), so fully green Renovate PRs (e.g. lint-staged update, all 9 checks passing) sat unmerged indefinitely with no visible error. nestjs-template had the setting on and merged fine, which made the inconsistency easy to miss.
• Updated the README to explain the tradeoff and how to switch back to platformAutomerge: true per-repo if the native queue (better branch-protection integration) is preferred once the setting is confirmed enabled everywhere.

Test plan

• Validated default.json is well-formed JSON (python3 -m json.tool)
• Confirm an already-green, currently-stuck Renovate PR in gardenia-api (e.g. #429) gets merged by Renovate on its next run without needing the repo setting change
• Confirm major-update PRs still do not automerge

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