Unified CI workflow for smarter PR gating

[alan-george-lk]

• Unifies all workflows under a top-level ci.yml workflow
  • This helps better visualize the entire workflow in a single link, all step summaries are viewable on a single page (e.g. clang-* results), and reduces run entries in the Actions repo tab
  • Also allows repo settings to reference one check for passing/failing PRs, and let's that workflow correctly dispatch which things to actually run
• Removes CPP_EXAMPLE_COLLECTION ref (duplicated across two files) and simply reads the submodule hash via git
• Attempt to use custom cache since sccache doesn't appear to be working
• Move out clang-* tools to cpp-checks.yml in case we want to move it into a separate repo for use across C++ projects
• Remove pin-check.yml, now natively supported in repo settings (turned on now)

[stephen-derosa]

a lot of magic going on, but if it fits it ships.

Consider drafting a pre-release off of this branch and explicitly targeting those artifacts in a manual cpp-example-collection build locally. Or at least the "Build against a local SDK" stage here