fix: prevent stale config migration during update

[dkmnx]

Summary

Stops kairo update from migrating provider models with stale defaults from the pre-update binary, which could rewrite configured models after an upgrade. This also adds regression coverage for the update flow and documents the fix in the changelog.

Changes

• remove post-install config migration from cmd/update.go
• inject update-step helpers so the update flow can be tested deterministically
• add a regression test ensuring kairo update leaves config.yaml unchanged after install
• document the updater fix in CHANGELOG.md

Test Plan

• go test ./cmd -count=1
• Verified the regression test covers preserving the configured model during update