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>

README.md

@@ -486,6 +486,10 @@ gateway flatly refuses to bind one with no keys at all.
 
 ## Development
 
+CI runs on code changes only — a README or `demo/` commit skips it. `[skip ci]`
+in a commit message skips it outright, and the arm64 image is only cross-built
+for tagged releases, since emulating it is most of the Docker job.
+
 ```bash
 ./scripts/demo.sh                     # scripted walkthrough against your providers
 node scripts/record-dashboard.mjs     # re-record the dashboard GIF