Feature Request: Granular control over workflow validation diagnostics

## Problem

The extension currently provides no user-configurable settings to control which validation diagnostics are shown in the Problems panel. Users cannot suppress specific categories of warnings/errors without disabling the extension entirely or changing file associations to plain YAML (losing all GitHub Actions language features).

## Current Behavior

The extension surfaces diagnostics like:
- `Context access might be invalid: ` (severity: hint)
- Expression validation errors
- Schema validation warnings

These are generated for valid workflow patterns (e.g., outputs set via `$GITHUB_OUTPUT` and referenced in subsequent jobs) that the static analyzer cannot fully resolve.

## Example

```yaml
jobs:
build:
outputs:
version: ${{ steps.version.outputs.DF_VERSION }}
steps:
- id: version
run: echo "DF_VERSION=$(cat VERSION)" >> $GITHUB_OUTPUT

deploy:
needs: build
steps:
- run: echo "Deploying ${{ needs.build.outputs.DF_VERSION }}"
# ⚠️ "Context access might be invalid: DF_VERSION"
```

The warning is a false positive—the workflow is correct, but the extension cannot trace the dynamic output.

## Requested Feature

Add granular settings to control validation behavior, for example:

```json
{
"github-actions.validation.enabled": true,
"github-actions.validation.severity": {
"context-access": "ignore", // or "hint" | "warning" | "error"
"unknown-action": "warning",
"deprecated-command": "hint",
"schema": "error",
"expression": "warning"
}
}
```

Alternatively, a simpler approach:

```json
{
"github-actions.validation.suppressedRules": [
"context-access",
"deprecated-command"
]
}
```

## Benefits

- Users can tailor validation to their workflow patterns
- Reduces noise from false positives without losing useful diagnostics
- Aligns with patterns in other linting extensions (ESLint, Pylint, etc.)

## Workaround (Current)

The only workaround is to change file associations:

```json
"files.associations": {
".github/workflows/*.yml": "yaml"
}
```

This disables **all** GitHub Actions features, which is undesirable.

## Environment

- Extension version: 0.29.1
- VS Code version: 1.96.x
- OS: macOS

Start by reviewing how the extension currently produces context-access, expression, schema, and other validation diagnostics, then compare those paths with the proposed settings. Done should include an agreed configuration model that lets users enable validation and control or suppress diagnostic categories without disabling GitHub Actions language features.

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