feat: Matrix sharding support (v1.3.0)

[Desperado]

Summary

Adds matrix sharding so large test suites can be split across parallel GitHub runners. Turns a 30-minute job into ~5 minutes with a simple matrix strategy.

Usage

```yaml
jobs:
test:
runs-on: ubuntu-latest
strategy:
fail-fast: false
matrix:
shard: [1, 2, 3, 4]
steps:
- uses: Quality-Max/qualitymax-github-action@v1
with:
api-key: ${{ secrets.QUALITYMAX_API_KEY }}
project-name: 'My Web App'
shard: ${{ matrix.shard }}
shards-total: 4
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
```

Changes

• action.yml — two new optional inputs (`shard`, `shards-total`)
• src/types.ts — `shard` / `shardsTotal` on ActionInputs
• src/index.ts
  • Parse inputs with validation (both must be positive ints, shard ≤ shardsTotal)
  • Append `--shard=N/M` to `npx playwright test` when sharding is active
  • Fix `totalTests` count — was using `scripts.length` (full suite) even for sharded runs. Now derived from actual parsed test results, so a shard running 25/100 tests reports 25, not 100.
  • Don't fabricate per-script result rows from exit code when sharded (we can't know which tests the shard actually ran without Playwright's JSON output)
• README — new "Matrix Sharding" example, inputs table updated
• Version — 1.2.1 → 1.3.0

Test plan

• Merge, cut v1.3.0 tag, update `v1` floating tag
• Trigger matrix workflow in qualitymax-demo-playground
• Verify each shard reports its own slice correctly

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