chore(upstream): auto-resolve conflicts with mergiraf + zdiff3 markers

[markijbema]

Summary

Automates most of the conflict-resolution grunt work during upstream merges and makes the leftovers dramatically easier to resolve by hand.

• Mergiraf (syntax-aware resolver): call mergiraf solve on each conflicted file before the kilocode-specific transform cascade in script/upstream/merge.ts. Auto-resolves the common pattern of neighbouring import additions around kilocode_change markers plus most JSON/YAML/TOML structural conflicts, so the existing i18n/branding/etc. transforms only see what's genuinely ambiguous. On a v1.14.28 merge run this resolved 45 of 74 conflicts.
• zdiff3 markers for the whole merge: pass merge.conflictStyle=zdiff3 to git merge in utils/git.ts, so every conflicted file carries base (|||||||) markers from the start — not just the ones mergiraf happens to touch. Manual resolution then shows what both sides changed relative to the common ancestor, which is much easier than reverse-engineering that from a 2-way marker.
• Never stage partial resolutions: mergiraf exit 2 means it reduced but didn't fully close the conflicts. The working tree keeps the reduced content (fewer markers to review), but the file is left unstaged so nothing half-resolved is auto-committed — you still see it in git status as needing attention.
• Resilient to per-file failures: every subprocess in runMergiraf uses .nothrow() and logs at debug level on non-zero exit, so a single problem file can't abort the entire merge mid-run.
• mergiraf required at startup: if missing the script aborts at step 1 with install instructions (brew / cargo / nix) rather than failing mid-merge.
• No .orig pile-up: pass --keep-backup=false to mergiraf solve so the working tree isn't littered with backup files afterwards.

Mergiraf is actively maintained (v0.16.3, 2026-01-26) and supports all the file types we hit during upstream merges (TS/TSX, JSON, YAML, TOML, etc.). Docs: https://mergiraf.org/.

Test plan

Run bun run script/upstream/merge.ts against an upstream version known to produce import-shuffle conflicts (e.g. anything recent where upstream moved imports around CrossSpawnSpawner / Config / Global). Verify:

1. Without mergiraf installed: script exits at step 1 with install instructions.
2. With mergiraf installed:
   • files with purely-additive import conflicts are auto-resolved and staged before the kilocode transform cascade runs;
   • the log reports both a full-resolution count (mergiraf auto-resolved N conflict(s)) and a partial-resolution count (mergiraf partially resolved M file(s) — remaining markers left unstaged for manual review);
   • files with real semantic conflicts still fall through to manual review — with zdiff3 markers showing the base version alongside ours/theirs.

Before:

After:

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• script/upstream/merge.ts
• script/upstream/utils/git.ts

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_{Reviewed by gpt-5.5-20260423 · 718,407 tokens}