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>

codecov.yml

@@ -0,0 +1,31 @@
+coverage:
+  status:
+    project:
+      default:
+        # Coverage can decrease by up to 1% and still pass
+        target: auto
+        threshold: 1%
+    patch:
+      default:
+        # New code should maintain coverage
+        target: auto
+
+comment:
+  # Post coverage comments on PRs (if we add PR coverage later)
+  behavior: default
+  layout: "header, diff, files"
+  require_changes: false
+
+# Ignore files that shouldn't affect coverage metrics
+ignore:
+  - "external/**"
+  - "ccan/**"
+  - "*/test/**"
+  - "tools/**"
+  - "contrib/**"
+  - "doc/**"
+  - "devtools/**"
+
+# Don't fail if coverage data is incomplete
+codecov:
+  require_ci_to_pass: false