ci: replace generated artifact-matrix workflows with one reusable + thin callers

[igorpecovnik]

What

Replaces the six generated complete-artifact-matrix-*.yml workflows (~9,400 lines each) and their generator with one hand-written reusable pipeline plus six thin per-track callers. No more code generation, no committed 9k-line giants, no 6× duplication.

Before	After
6 generated workflows (~56k lines)	6 thin callers (~60 lines each)
recreate-matrix.yml (600-line bash/awk generator)	— (deleted)
8 Jinja chunk templates + 8 gha_config_*.yaml	complete-artifact-matrix.yml + 2 reusable chunk bodies
watchdog.yml (15-min cron, reruns failed jobs)	retry-on-failure inlined in the build steps

2,160 lines total, hand-maintained.

How it works

• complete-artifact-matrix.yml — reusable (workflow_call) machinery: team_check → version_prep → matrix_prep → {artifact chunks} → {image chunks} → publish-debs-to-repo → closing.
• build-artifacts-chunk.yml / build-images-chunk.yml — the per-chunk build bodies, written once and fanned out over a chunk: [1..17] / [1..16] matrix that indexes matrix_prep's named outputs (outputs[format('artifacts-chunk-json-{0}', matrix.chunk)]).
• 6 callers (build-nightly, build-standard-support, build-community, build-apps, build-all, build-all-stable) — each holds its own schedule, concurrency, dispatch inputs, and a single uses: with the track's values.

Per-track behaviour (schedules, repos, upload paths, menu defaults, the CHECK_OCI-uses-nightlybuild-default quirk) is preserved. The empty all/all-stable board dropdown (a missing-file bug in the old generator) is gone; use targetsFilterInclude to scope a board.

Targets (targets-release-*.yaml) are already fetched at runtime from github.armbian.com/release-targets/, so board classification / targets generation no longer lives in this repo. targets-release-*.yaml, the templates, and complete-artifact-one-by-one.yml are left untouched.

watchdog.yml is dropped; both build steps now retry the compile.sh invocation BUILD_ATTEMPTS (default 3) times with backoff. watchdog's entry list moved with it.

Validation

• actionlint clean (custom self-hosted runner labels declared in .github/actionlint.yaml).
• The matrix + reusable-workflow dynamic-output-indexing mechanic was smoke-tested on real GitHub Actions (temporary push-triggered workflow, since removed): chunk 1 fanned out into its matrix items via the dynamic index, the empty chunk was correctly skipped.
• Not yet exercised: a real compile.sh gha-matrix run on the super self-hosted runner with NetBox secrets — that needs the workflows on the default branch (workflow_dispatch requirement). The first nightly/dispatch after merge is the true end-to-end check. If the dynamic index ever misbehaves at scale, the fallback is a static matrix.include enumeration (same structure).