Run CI only for examples changed in a PR

[dhruvja]

Currently, CI runs broadly even when a PR only updates a single example. This makes CI slower and noisier than necessary.

We should update the CI workflow so it detects which example directories were changed and only runs the relevant checks for those examples.

Expected behavior

• If spl-tokens/** changes, run CI for spl-tokens only.
• If anchor-counter/** changes, run CI for anchor-counter only.
• If multiple examples change, run CI for each changed example.
• If shared scripts, root config, or workflow files change, run the full CI matrix.
• Documentation-only changes should skip example builds/tests where safe.

Why

• Reduces CI runtime.
• Avoids unrelated example failures blocking focused PRs.
• Makes PR feedback easier to understand.