ci: path-filter the pull_request trigger too

The push trigger was already restricted to master and to source paths,
but pull_request carried only a branch filter — so a PR touching nothing
but the README would have run a full build. A branch filter constrains
where a workflow fires, not what fires it.

Both filters now list the same six paths. A docs-only change runs nothing,
on either trigger.

Note that .github/workflows/** is deliberately absent from both lists, so
editing this file does not run it. That is the intended trade: fewer
pointless builds, at the cost of a broken workflow surfacing on the next
source change rather than immediately.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHt4DykxXaQnyaJfyS8KCn

.github/workflows/ci.yaml

@@ -10,8 +10,17 @@ on:
       - 'Cargo.lock'
       - 'bundler.toml'
       - 'tools/**'
+  # Mirrors the push filter. Without it a PR touching only the README would run a
+  # full build - the branch filter alone constrains where, not what.
   pull_request:
     branches: [ master ]
+    paths:
+      - 'src/**'
+      - 'xtask/**'
+      - 'Cargo.toml'
+      - 'Cargo.lock'
+      - 'bundler.toml'
+      - 'tools/**'
   workflow_dispatch:
 
 # Target the self-hosted Rust/audio runner registered to the `musiikki` org.