Skip CI builds for PRs that only touch build/test-irrelevant files

[alexreinking]

Summary

• Adds a fileIsImportant predicate to the halide-main scheduler that skips scheduling builds when a PR's changed files are all drawn from a known-safe set that can't affect this buildbot's CMake configure/build/ctest outcome: docs (*.md, doc/**), .github/**, the legacy Makefile-based build (never invoked by this buildbot), unreachable demo apps (apps/Hello*), and dev-only tools/ lint/debugger scripts. Full list and rationale in the diff comments.
• Deliberately does not exclude apps/images/**, apps/vcpkg/**, tools/launch_wasm_test.js, or python_bindings/**/pyproject.toml — verified these are load-bearing for this pipeline (sample images read by CMake-built apps, vcpkg overlay ports for apps/hannk, wasm generator test launcher, and python bindings actually built/tested here).
• Fails open: an empty/missing changed-files list (e.g. a rate-limited GitHub API call) is always treated as important, so it never silently skips a real change.
• Adds token=GITHUB_TOKEN to the GitHub webhook's change_hook_dialects config so the PR-files API lookup buildbot already performs is authenticated, reducing the chance of hitting the unauthenticated rate limit.

Test plan

• python3 -c "import ast; ast.parse(...)" — config parses cleanly
• buildbot checkconfig . — full config loads successfully
• Standalone assertions against _is_ignorable_path/is_change_important covering ignorable paths (README.md, .github/workflows/*.yml, legacy Makefiles, apps/HelloWasm/*), non-ignorable paths (src/*.cpp, apps/images/*, apps/vcpkg/*, tools/launch_wasm_test.js, python_bindings/*, CMakeLists.txt), mixed-file PRs, and the empty-file-list fail-open case — all passed
• Observe a real PR touching only docs/.github to confirm no build is scheduled, and a real PR touching source to confirm builds still run as expected

🤖 Generated with Claude Code