chore(skills): add repository-local workflow skills (#2190)

.agents/skills/test-coverage-improver/SKILL.md

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+---
+name: test-coverage-improver
+description: Run project coverage checks, rank high-risk gaps, and propose high-impact tests to improve regression confidence for changed and critical code paths before release.
+---
+
+# Test Coverage Improver
+
+Use this skill when you need a prioritized, risk-aware plan to improve tests from coverage results.
+
+## Usage assumptions
+- Focus scope is either changed lines/files, a module, or the whole repository.
+- Coverage artifact must be generated or provided in a supported format.
+- If required context is missing, call out assumptions explicitly before proposing work.
+
+## Workflow
+
+1. Define scope and baseline
+   - Confirm target language, framework, and branch.
+   - Confirm whether the scope is changed files only or full-repo.
+
+2. Produce coverage snapshot
+   - Rust: `cargo llvm-cov` (or `cargo tarpaulin`) with existing repo config.
+   - JavaScript/TypeScript: `npm test -- --coverage` and read `coverage/coverage-final.json`.
+   - Python: `pytest --cov=<pkg> --cov-report=json` and read `coverage.json`.
+   - Collect total, per-file, and changed-line coverage.
+
+3. Rank highest-risk gaps
+   - Prioritize changed code, branch coverage gaps, and low-confidence boundaries.
+   - Apply the risk rubric in [coverage-prioritization.md](references/coverage-prioritization.md).
+   - Keep shortlist to 5–8 gaps.
+   - For each gap, capture: file, lines, uncovered branches, and estimated risk score.
+
+4. Propose high-impact tests
+   - For each shortlisted gap, output:
+     - Intent and expected behavior.
+     - Normal, edge, and failure scenarios.
+     - Assertions and side effects to verify.
+     - Setup needs (fixtures, mocks, integration dependencies).
+     - Estimated effort (`S/M/L`).
+
+5. Close with validation plan
+   - State which gaps remain after proposals.
+   - Provide concrete verification command and acceptance threshold.
+   - List assumptions or blockers (environment, fixtures, flaky dependencies).
+
+## Output template
+
+### Coverage Snapshot
+- total / branch coverage
+- changed-file coverage
+- top missing regions by size
+
+### Top Gaps (ranked)
+- `path:line-range` | risk score | why critical
+
+### Test Proposals
+- `path:line-range`
+  - Test name
+  - scenarios
+  - assertions
+  - effort
+
+### Validation Plan
+- command
+- pass criteria
+- remaining risk