Stop running the full pipeline on documentation commits

A README change was building a multi-arch Docker image. Both workflows now
ignore markdown, docs/, demo/, .gitignore and LICENSE. providers/*.json is
deliberately not ignored: CI fails if one stops parsing, which is the one data
change that can break a running gateway.

arm64 is cross-built under QEMU and dominates the Docker job, so it is now only
built for tagged releases; main and PRs build native amd64 only. QEMU setup is
skipped entirely when it is not needed.

Anchors would have kept the paths-ignore lists DRY, but Actions has been
unreliable with them and a broken workflow costs more than five duplicated
lines.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -3,7 +3,21 @@ name: CI
 on:
   push:
     branches: [main]
+    # Prose cannot break a build. Provider JSON can, so it is NOT ignored —
+    # the last step here fails if any providers/*.json stops parsing.
+    paths-ignore:
+      - '**/*.md'
+      - 'docs/**'
+      - 'demo/**'
+      - '.gitignore'
+      - 'LICENSE'
   pull_request:
+    paths-ignore:
+      - '**/*.md'
+      - 'docs/**'
+      - 'demo/**'
+      - '.gitignore'
+      - 'LICENSE'
   workflow_dispatch:
 
 concurrency: