Reconcile platform updates with one merge

[hamzamerzic]

Summary

• reconcile diverged platform histories by merging the exact reviewed update once instead of replaying every local commit
• preserve local commit identities and surface the complete net conflict set together, while retaining fast-forward, serve-old, import-probe, and frontend rollback safeguards
• bind manual conflict repair to the immutable reviewed commit and retain cleanup for interrupted updates from older builds

Why

An instance with many local platform commits could spend most of an update replaying them one at a time. Conflicts then appeared sequentially, forcing repeated repair cycles even when they belonged to the same reviewed update. A single net merge makes the work proportional to the current divergence rather than the number of historical local commits.

Validation

• 66 focused platform-update tests passed, including timeout and generic no-path merge failures; the surrounding route and recovery checks remain covered
• 22 frontend watcher publication tests passed
• syntax, shell parsing, full-diff privacy/metadata checks, and diff checks passed
• the real 25-local-commit pre-update history reached one bounded merge attempt in 0.45 seconds and surfaced all 15 net conflict paths together
• full host-side backend run: 3,076 passed and 1 skipped; six environment-only failures were isolated to missing linked-worktree frontend dependencies (the affected tests passed when provided) and build-sentinel tests that require the repository's isolated container runner