Migrate near-sandbox nearcore updates to Renovate

[r-near]

Replace the custom check-nearcore-release polling scripts in both sandbox repos with Renovate release tracking. The custom automation silently broke in near-sandbox-js (regex drift after a formatter change + GitHub auto-disabling the scheduled workflow), motivating the move to the Renovate app. The app is now approved and active on the org.

PRs in review

• chore: replace nearcore updater with Renovate near-sandbox-rs#83 — Renovate config for src/lib.rs; removes the legacy updater
• ci: add Renovate for nearcore updates with automatic changesets near-sandbox-js#53 — Renovate config for src/constants.ts + workflow that amends a changeset into Renovate's bump commits so changeset publish cuts releases; removes the legacy updater

Follow-ups after merge

• Confirm the Dependency Dashboard on each repo lists near/nearcore from the custom regex manager
• Verify Renovate's first bump PR on each repo (and the changeset-amend workflow on the js side)