feat: port TagTracker custom overlay to current upstream and drop Herelink

[DonLakeFlyer]

Ports the custom/ overlay and CI to the upstream qgroundcontrol master merged in the sync-upstream-master baseline commit, and drops Herelink support. The baseline branch does not build on its own; this PR is what makes it build.

The diff touches only fork-owned paths: custom/, .github/, .gitmodules, README.md. Zero changes to upstream files.

Custom overlay

• Replace .qrc/exclusion resource machinery with qt_add_resources + qt_add_qml_module (QGroundControl.CustomControls) per the new custom-build contract (CUSTOM_SOURCES/LIBRARIES/INCLUDE_DIRECTORIES/DEFINITIONS)
• Add CustomOverrideInterceptor so qrc:/qml/<path> resolves to the custom override under /Custom/qml when one exists
• Update QML imports for renamed/removed upstream modules and drop version numbers

State machine

• Rebase CustomState/CustomStateMachine on upstream QGCState/QGCStateMachine
• Remove duplicated DelayState/FunctionState/SendMavlinkCommandState in favour of the upstream implementations

Settings

• Register CustomSettings through QGCCorePlugin::registerCustomSettings and expose it as settingsManager.customSettings
• Add the TagTracker settings page via the SettingsPages.json overlay
• Add required label metadata to Custom.SettingsGroup.json

Behaviour moved out of upstream files into supported hooks

• Default-enable CustomPluginLog in CustomPlugin::init()
• EMERGENCY STOP as a toolbar indicator instead of a FlyViewToolBar patch

Removed

• Herelink core plugin/options and the TAG_TRACKER_HERELINK_BUILD conditionals
• Stale custom/android manifest and build.gradle overlay (upstream now covers it)
• tunnel-protocol git submodule; fetched via CPMAddPackage instead

CI

• Rewrite macOS and Android workflows on the upstream composite actions; artifacts via upload-artifact, no S3/attestation/Play Store
• Trigger on sync-upstream-master; cancel-in-progress only for pull requests
• The large .github line count is upstream's actions/ + scripts/ brought in verbatim because the new workflows depend on them

Not yet done

• ctest has not been run
• Android has not been built locally; relying on the CI run here

Refs mavlink/qgroundcontrol#15026