docs: workflow-architecture.md needs updates for recent changes

## Problem

`docs/src/ci/workflow-architecture.md` doesn't reflect recent workflow changes.

## Issues Found

### 1. Missing Auto-Merge Workflow
No mention of `auto-merge-integration.yaml` which handles:
- Trust-based auto-merge for integration PRs
- Dependabot trust path
- CODEOWNERS verification
- Commit signature verification

### 2. Missing repository_dispatch Trigger
W6 (`release-atomic-chart.yaml`) now supports `repository_dispatch` trigger for secure manual releases. This isn't documented.

### 3. Missing ci/dependencies Branch
W2 now has a `process-dependencies` job that routes dependabot workflow updates to a `ci/dependencies` atomic branch. Not documented.

### 4. Branch Naming Convention Incomplete
Doc mentions `charts/` and `integration/` patterns but doesn't mention `ci/dependencies`.

### 5. Workflow File Names May Be Outdated
Need to verify the workflow file names match what's actually in `.github/workflows/`.

## Recommended Changes

1. Add section on auto-merge workflow
2. Document repository_dispatch trigger for W6
3. Add ci/dependencies branch to branch naming section
4. Update any stale workflow file references
5. Add diagram showing auto-merge integration

## Related
- Part of documentation refresh for chart release workflows