ci: skip jobs a pull request can't affect and bound every job's run time (#1501)

All workflows share the account's 20 concurrent hosted runners, and each PR push
queued 14 jobs whatever it touched, so releases and other PRs waited behind them.

- CI: a Changes job lists the PR's files. Rust (3 OSes), the musl server, plugins
  and cargo deny skip when only frontend, test or docs files changed (files the
  Rust code embeds from src/ still count as Rust); Frontend and E2E skip on
  docs-only changes. Pushes and manual runs still run everything.
- Language smoke: pushes to main use the same path filter as pull requests.
- Every CI job has a timeout, so a hung job can't hold a runner for 6 hours.
- CI's token is read-only.

.github/workflows/ci.yml

@@ -7,6 +7,9 @@ on:
     types: [opened, synchronize, reopened]
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 concurrency:
   group: ci-${{ github.ref }}
   # A newer push supersedes the run in progress, on main too.
@@ -19,8 +22,50 @@ env:
   CARGO_PROFILE_DEV_DEBUG: line-tables-only
 
 jobs:
+  # Which parts a pull request touches, so jobs it can't affect skip and leave the
+  # shared runners to others. Pushes and manual runs always run everything.
+  changes:
+    name: Changes
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      pull-requests: read
+    outputs:
+      rust: ${{ steps.classify.outputs.rust }}
+      frontend: ${{ steps.classify.outputs.frontend }}
+    steps:
+      - id: classify
+        env:
+          GH_TOKEN: ${{ github.token }}
+          PR: ${{ github.event.pull_request.number }}
+        run: |
+          set -euo pipefail
+          if [ "$GITHUB_EVENT_NAME" != pull_request ]; then
+            echo "rust=true" >> "$GITHUB_OUTPUT"
+            echo "frontend=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          gh api --paginate "repos/$GITHUB_REPOSITORY/pulls/$PR/files" \
+            --jq '.[] | .filename, (.previous_filename // empty)' > files.txt
+          cat files.txt
+          docs='^(docs/|\.agents/|\.claude/|\.idea/|\.github/ISSUE_TEMPLATE/|\.github/pull_request_template\.md$|[^/]+\.md$|LICENSE$|NOTICE$)'
+          # Frontend-only paths. The Rust code embeds a few files under src/; those stay out.
+          web='^(src/|e2e/|perf/|public/|scripts/|smoke/|index\.html$|vite\.config\.ts$|tsconfig[^/]*\.json$|knip\.ts$|playwright\.config\.ts$|\.oxlintrc\.json$|\.oxfmtrc\.json$)'
+          embedded='^src/(bindings\.ts|styles/tokens\.css|features/plugins/views/frame-bridge\.js|platform/keymaps/)'
+          # A job runs when any changed file falls outside the paths that can't affect it.
+          rust=false
+          if grep -Ev "$docs|$web" files.txt | grep -q . || grep -Eq "$embedded" files.txt; then rust=true; fi
+          frontend=false
+          if grep -Ev "$docs" files.txt | grep -q .; then frontend=true; fi
+          echo "rust=$rust frontend=$frontend"
+          echo "rust=$rust" >> "$GITHUB_OUTPUT"
+          echo "frontend=$frontend" >> "$GITHUB_OUTPUT"
+
   rust:
     name: Rust (${{ matrix.os }})
+    needs: changes
+    if: needs.changes.outputs.rust == 'true'
+    timeout-minutes: 60
     strategy:
       fail-fast: false
       matrix:
@@ -62,6 +107,9 @@ jobs:
   # keep that build working.
   server:
     name: Remote server (linux musl)
+    needs: changes
+    if: needs.changes.outputs.rust == 'true'
+    timeout-minutes: 40
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v5
@@ -77,6 +125,9 @@ jobs:
   # own workspaces (the root excludes them), so the jobs above don't see them.
   plugins:
     name: Plugins (SDK, official, fixtures)
+    needs: changes
+    if: needs.changes.outputs.rust == 'true'
+    timeout-minutes: 30
     runs-on: ubuntu-22.04
     env:
       CARGO_TARGET_DIR: ${{ github.workspace }}/target-plugins
@@ -126,6 +177,9 @@ jobs:
 
   deny:
     name: Cargo deny
+    needs: changes
+    if: needs.changes.outputs.rust == 'true'
+    timeout-minutes: 15
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
@@ -135,6 +189,9 @@ jobs:
 
   frontend:
     name: Frontend
+    needs: changes
+    if: needs.changes.outputs.frontend == 'true'
+    timeout-minutes: 30
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v5
@@ -152,6 +209,9 @@ jobs:
 
   e2e:
     name: E2E
+    needs: changes
+    if: needs.changes.outputs.frontend == 'true'
+    timeout-minutes: 30
     runs-on: ubuntu-24.04
     # Browsers and their system libraries are preinstalled; keep the tag in step with @playwright/test.
     container: mcr.microsoft.com/playwright:v1.64.1-noble

.github/workflows/language-smoke.yml

@@ -11,10 +11,7 @@ name: Language smoke
 on:
   push:
     branches: [main]
-  pull_request:
-    branches: [main]
-    types: [opened, synchronize, reopened]
-    paths:
+    paths: &paths
       - "crates/**"
       - "languages/**"
       - "fixtures/**"
@@ -27,6 +24,10 @@ on:
       - "package.json"
       - "pnpm-lock.yaml"
       - ".github/workflows/language-smoke.yml"
+  pull_request:
+    branches: [main]
+    types: [opened, synchronize, reopened]
+    paths: *paths
   workflow_dispatch:
 
 concurrency: