Limit clang-tidy CI to changed files on branch workflows

[simon-d-bmw]

Summary

• run the full clang-tidy scan only on pushes to main
• run clang-tidy on changed .c, .cc, and .cpp files for pull requests, merge groups, and other branch pushes
• skip clang-tidy cleanly when no C/C++ files changed

Details

• add a dedicated CI helper to determine clang-tidy scope from the GitHub event payload
• keep the existing clang-tidy findings export and reporting flow
• treat an empty clang-tidy findings YAML as zero findings so filtered runs do not fail when no diagnostics are emitted
• update the developer documentation to describe the branch-vs-main behavior

Refs #381

[simon-d-bmw]

CI scope validation campaign summary (branch-event changed-scope behavior):

• Validation completed across 8 scenarios with posix + s32k1xx matrix jobs.
• Overall result: successful; no workflow crashes in scope or resolver path.
• S01 is considered a scenario-selection limitation (changed header not included in analyzed target set), not a resolver mismatch.
• S08 confirms the mapped-header positive case (header-only change resolved to many TUs on both matrix jobs).
• S07 (merge-history) passed with merge commit + follow-up commit.

Validation PR references (all test PRs now closed):

• S01: ci-scope S01: header-only scenario for clang-tidy CI #391
• S02: ci-scope S02: header+source scenario for clang-tidy CI #392
• S03: ci-scope S03: source rename scenario for clang-tidy CI #393
• S04: ci-scope S04: header rename scenario for clang-tidy CI #397
• S05: ci-scope S05: source deletion scenario for clang-tidy CI #398
• S06: ci-scope S06: non-C/C++-only scenario for clang-tidy CI #399
• S08: CI scope scenario S08: header-only change on mapped platform header #400
• S07: CI scope scenario S07: merge-history branch #401