fix(migration): skip backup when config content is unchanged (fixes #3222)

[MoerAI]

Summary

• Prevent unnecessary .bak.<timestamp> backup files on every startup when no config changes are needed

Problem

migrateConfigFile() sets needsWrite=true in several code paths that update migration tracking state (sidecar writes, stripping legacy _migrations field) without actually changing the config file body. When needsWrite is true, a backup file is unconditionally created and the config is rewritten, even when the output is identical to the existing file. This causes a new .bak.<timestamp> file to accumulate on every OpenCode startup.

Fix

Added a content comparison before the backup/write block. The function now reads the existing config file content and compares it against the serialized migration output. If they are identical, the function applies in-memory mutations (e.g. stripped _migrations) and returns false without touching the filesystem. Backup and rewrite only occur when the config content has actually changed.

Changes

File	Change
src/shared/migration/config-migration.ts	Add content comparison guard before backup creation

Fixes #3222

---

Summary by cubic

Skip creating .bak.<timestamp> files and rewrites during config migration when the migrated output matches the original in-memory config. This prevents backup sprawl on startup and avoids unnecessary disk I/O.

• Bug Fixes
  • Compare the serialized migrated config to an originalSnapshot of rawConfig (in-memory) before backup/write.
  • If identical, mutate rawConfig in memory and return; only create a backup and rewrite when content changes.

^{Written for commit dc62243. Summary will update on new commits.}

[MoerAI]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Prevents redundant file writes and backups by checking for content changes. Safely updates in-memory config state and returns false correctly when no disk I/O occurs.}

[cubic-dev-ai]

0 issues found across 1 file (changes from recent commits).

_{Auto-approved: Prevents redundant backups by comparing serialized config before writing. Correctly handles in-memory object updates and uses standard JSON comparison to avoid unnecessary disk I/O.}