chore(ai): add testing-gaps skill

Author: JamieMason

.claude/skills/testing-gaps/SKILL.md

@@ -0,0 +1,99 @@
+---
+name: testing-gaps
+description: Run coverage, inspect results, and identify missing test scenarios for a given source file. Use when analysing test coverage or finding untested branches.
+---
+
+# Testing Gaps
+
+Find untested branches and missing real-world test scenarios for a source file.
+
+## Workflow
+
+1. **Run text coverage** for the target file
+2. **Identify uncovered branches** from the output
+3. **Cross-reference** with test file to understand what's tested
+4. **Report** uncovered branches + missing real-world scenarios
+
+## Step 1: Run Coverage
+
+Use `cargo llvm-cov` with `--text` output (not `--html`) for parseable results.
+
+```bash
+# Full coverage, filtered to target file
+cargo llvm-cov test --text \
+  --ignore-run-fail \
+  --ignore-filename-regex '(_test.rs|\/test\/)' \
+  2>/dev/null | sed -n '/TARGET_FILE\.rs:/,/^$/p'
+```
+
+Example for `preferred_semver.rs`:
+
+```bash
+cargo llvm-cov test --text \
+  --ignore-run-fail \
+  --ignore-filename-regex '(_test.rs|\/test\/)' \
+  2>/dev/null | sed -n '/preferred_semver\.rs:/,/^$/p'
+```
+
+## Step 2: Read Coverage Output
+
+The text format shows per-line execution counts:
+
+```
+  42|     80|        .and_then(|range| ...)     # Hit 80 times
+  74|      0|        Some(preferred)            # Never hit
+ 321|      0|        instance.mark_conflict(...) # Never hit
+```
+
+- `count > 0` = covered
+- `count = 0` = uncovered branch
+- `^N` annotations on sub-expressions show partial coverage within a line
+
+### Extract only uncovered lines
+
+```bash
+... | grep -E '^\s+\d+\|\s+0\|'
+```
+
+## Step 3: Classify Gaps
+
+For each uncovered branch, determine:
+
+1. **What code path leads here?** — Trace the `if/else` chain backward
+2. **What input would trigger it?** — What package.json + config combination
+3. **Is it reachable?** — Some branches may be defensive/unreachable
+4. **Is it worth testing?** — Real-world scenario vs theoretical edge case
+
+### Categories
+
+| Category                         | Action            |
+| -------------------------------- | ----------------- |
+| Real-world scenario never tested | Write a test      |
+| Edge case in existing logic      | Write a test      |
+| Defensive branch (unreachable)   | Note but skip     |
+| Dead code                        | Consider removing |
+
+## Step 4: Identify Missing Scenarios
+
+Beyond line coverage, look for missing _combinations_:
+
+- Feature A tested, Feature B tested, A+B never tested together
+- Only tested with one package, never with multiple
+- Only tested with `highestSemver`, never `lowestSemver`
+- Only tested with one range type (`^`), never others (`>=`, `<=`)
+- Only tested without semver groups, never with
+
+## Step 5: Report
+
+Structure findings as:
+
+1. **Uncovered branches** — Specific lines, what triggers them, why untested
+2. **Missing real-world scenarios** — Combinations and interactions not covered
+
+## Running Tests After Adding Coverage
+
+```bash
+just test                              # All tests
+cargo test test_name -- --nocapture   # Specific test
+just coverage                         # Regenerate coverage report
+```