PROD-2203: Add --preflight (dry-run preview) mode to sync/push

[5PK]

What & why

Implements PROD-2203. Adds a preflight mode to the sync/push command that previews what a sync would do — before committing any changes to the target.

Preflight runs the full source-pull, target-pull, dependency analysis and change detection that a real sync performs, then reports the creates / updates / skips / conflicts it would produce — without writing anything to the target instance or mapping files. This lets customers and support preview sensitive migrations safely.

Flags

• --preflight — run the preview (no writes to target or mappings)
• --preflight-json — emit the report as machine-readable JSON instead of the human table

Exits non-zero when conflicts are detected, so CI/scripts can gate a real sync on a clean preflight.

How it works

The key design point: skip and conflict decisions never reach the API (the sync decides "nothing to do" / "unsafe" and moves on), so a client-level interceptor can't observe them. Each pusher therefore records its already-computed decision at the decision point and skips the write.

• New PreflightReport collector (src/lib/preflight/) — per-phase counts + table/JSON rendering
• All 7 pushers instrumented: galleries, assets, models, containers, content, templates, pages
• saveMappingFile hard-guarded under preflight (safety net); auto-publish suppressed
• Previously-silent "target changed without --overwrite" fall-throughs (galleries/assets) are now surfaced as conflicts in the report

Sample output

🔎 PREFLIGHT — no changes were written to the target or mappings
Models: 2 create, 1 update, 5 skip, 0 conflict
Pages: 0 create, 3 update, 1 conflict
  ✗ [en-us] Home — target changed; use --overwrite to force
TOTAL: 2 create, 4 update, 6 skip, 1 conflict
⚠️  Conflicts detected — a real sync would require --force (exit code 1).

Testing

• New unit tests for the report module + the saveMappingFile preflight guard
• Full suite: 1743 tests passing (98 suites); tsc --noEmit clean; npm run build passes

Notes for reviewers

• Related ticket PROD-2202 (move model validation to first sync step) will affect what the models phase reports under preflight, depending on merge order.
• Suggest a quick live --preflight run against a throwaway target to eyeball report formatting (automated tests cover logic, not a live sync).

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