[Feature] Support multi-source Applications in repo.GetCommitMetadata

## Summary

The template helper `repo.GetCommitMetadata(revision)` reads `application.spec.source.repoURL` to determine which repo to query. For multi-source ArgoCD Applications (where `spec.source` is empty and the source list lives at `spec.sources[]`), the helper fails with:

```
error calling call: failed to get application source repo URL
```

This kills the entire notification template render — no message is delivered.

## Repro

ArgoCD ≥ 2.6 supports multi-source Applications. Example spec:

```yaml
spec:
sources:
- repoURL: https://github.com/org/gitops.git
path: chart
helm:
valueFiles:
- $values/envs/prod.yaml
- repoURL: https://github.com/org/gitops.git
ref: values
```

Notification template using:

```
{{ $commit := call .repo.GetCommitMetadata (index .app.status.sync.revisions 0) }}
{{ $commit.Author }}
```

fails to render. The same App displays commit metadata correctly in the ArgoCD UI, which uses `ApplicationService.RevisionMetadata` with a `sourceIndex` parameter.

## Proposed Fix

Two options, not mutually exclusive:

**Option A — make `GetCommitMetadata` fall back to `sources[0]`** (backwards-compatible, fixes most users with no template changes):

```go
func GetCommitMetadata(revision string) (CommitMetadata, error) {
repoURL := app.Spec.Source.RepoURL
if repoURL == "" && len(app.Spec.Sources) > 0 {
repoURL = app.Spec.Sources[0].RepoURL
}
// ... existing lookup
}
```

**Option B — add a sibling helper that takes a source index** (explicit, handles non-zero indices):

```go
func GetCommitMetadataFromSource(sourceIndex int, revision string) (CommitMetadata, error)
```

Template usage:

```
{{ call .repo.GetCommitMetadataFromSource 0 (index .app.status.sync.revisions 0) }}
```

## Impact

Anyone using the standard `app-deployed` / `app-sync-failed` / `app-health-degraded` templates with multi-source Applications loses commit author/message in Slack/email. Affects GitOps setups that use the chart-source + values-source split, which is common since ArgoCD 2.6.

Start at the repo.GetCommitMetadata entry point and compare it with ArgoCD's ApplicationService.RevisionMetadata, which accepts a sourceIndex. Review the standard app-deployed, app-sync-failed, and app-health-degraded templates to understand current usage. Done means multi-source Applications render commit metadata without breaking existing single-source behavior, with support for the chosen fallback or indexed helper design.

Written by the indexing model from the issue text.