uSync: forward-port v17→v18, CI parity, v17.4.2 and v18.1.4 releases

Repo: KevinJump/uSync

Fixed 'clean' delete-missing-items ignoring handler import rules (e.g. ExcludeContentTypes) — a partial sync from uSync.Publisher could delete items it never sent (#1092)
Reviewed and merged #1089 (honour ExportAtStartup group) and #1091 (build History client assets before packaging)
Triggered a nightly pre-release build from v17/main
Released v17.4.2 to NuGet
Forward-ported v17's #1089 (honour ExportAtStartup group), #1092 (clean respects import rules), and #1091's History-client CI build to v18, as #1094, #1095, and #1093
Brought v17's and v18's CI workflows into parity (action versions, global.json SDK pinning, restructured nightly-build pipeline) — #1096
Triggered pre-release builds on both branches to confirm the CI changes work end-to-end
Released v18.1.4 to NuGet