feat: Remove non-matrix mode, always generate single validate job

[jdutton]

Summary

• Removes non-matrix CI workflow generation mode entirely — generate-workflow now always produces a single validate job with matrix strategy
• This matches the local pnpm validate experience: all steps run sequentially in one runner, so setup steps (e.g., npx playwright install) execute before tests
• Eliminates the useMatrix option, getAllJobIds(), createCommonJobSetupSteps(), and addCoverageReportingSteps() (-434 lines net)
• Projects previously using non-matrix mode will see doctor report workflow out-of-sync and can regenerate with npx vibe-validate generate-workflow

Test plan

• All 43 generate-workflow tests pass (removed 17 non-matrix tests, updated remaining)
• Full pnpm validate passes (build, typecheck, lint, duplication, unit tests, integration tests)
• CLI reference docs regenerated and in sync
• Version bumped to v0.19.0-rc.13

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.80851% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.72%. Comparing base (8ef62e2) to head (822ce8a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/cli/src/commands/generate-workflow.ts	96.80%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   85.79%   85.72%   -0.08%     
==========================================
  Files         115      115              
  Lines       15068    14903     -165     
  Branches     3245     3215      -30     
==========================================
- Hits        12928    12775     -153     
+ Misses       2140     2128      -12     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud