ci(actions): add composite actions and clang-tidy PR helper

[mrpollo]

Adds four reusable CI building blocks that upcoming optimization PRs will consume. No existing workflow is modified, so this change is dormant on merge: grep -r 'setup-ccache\|save-ccache\|build-gazebo-sitl\|run-clang-tidy-pr' .github/workflows/ returns nothing. CI behavior on main is unchanged by this PR.

Context: this is the foundation PR for a 6-PR series that back-ports the individual optimizations from #26257 (the orchestrator consolidation) without adopting the orchestrator architecture itself. Each follow-up PR will start referencing these files.

• .github/actions/setup-ccache restores ~/.ccache with content-hash keys (prefix-ref-sha with ref and base_ref fallbacks), installs ccache optionally, and writes ccache.conf with compression, hash_dir = false, and compiler_check = content.
• .github/actions/save-ccache prints stats and saves the cache under the primary key produced by setup-ccache, using if: always() so it runs on failures too.
• .github/actions/build-gazebo-sitl builds px4_sitl_default plus the Gazebo Classic plugins and prints ccache -s between the two stages. Used by ros_integration_tests.yml in a later PR.
• Tools/ci/run-clang-tidy-pr.py diffs the PR against its base, maps changed .cpp/.c files to translation units in compile_commands.json, grep-maps changed headers to TUs that include them, and invokes Tools/run-clang-tidy.py only on that subset. Exits 0 silently when no C++ files changed or no TUs matched. Will be wired into clang-tidy.yml in a later PR.

Nothing imports these yet, so the blast radius is zero. Verification: no workflow references them, the Python script is self-contained with python3 stdlib only, and python3 -m py_compile Tools/ci/run-clang-tidy-pr.py is clean.