Add comprehensive coverage infrastructure with clang source-based coverage

This commit introduces a modern coverage infrastructure for Core Lightning:

- Migrate from ad-hoc coverage script to integrated Makefile targets
- Add LLVM source-based coverage support with per-test profraw organization
- Integrate coverage collection into pytest framework via TailableProc
- Add GitHub Actions workflow for nightly coverage reports
- Add Taskfile.yml for convenient task automation
- Add codecov.yml for Codecov integration
- Add comprehensive coverage documentation in COVERAGE.md
- Update contributor workflow docs with new coverage script path
- Add coverage data files to .gitignore (*.profraw, *.profdata)
- Remove obsolete contrib/clang-coverage-report.sh
- Remove obsolete tests/conftest.py (now using pyln-testing markers)
- Update pyproject.toml to include pyln-testing in main dependencies

The new infrastructure automatically collects coverage data when CLN_COVERAGE_DIR
is set, organizing profraw files by test name for granular analysis.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

pyproject.toml

@@ -16,8 +16,9 @@ dependencies = [
   "pyln-proto",
   "pyln-grpc-proto",
   "pytest-trackflaky",
-  "pyln-testing",
   "pytest-rerunfailures>=16.0.1",
+  "pyln-testing",
+  "pyln-proto",
 ]
 package-mode = false
 [dependency-groups]