feat: add inline image response mode for broader MCP client compatibility

## Summary

Shellwright currently returns download URLs for screenshots/recordings, requiring agents to have filesystem access (Bash + file read). This limits compatibility to agents like Claude Code.

Adding an optional inline mode would enable Shellwright to work with pure MCP clients like Claude Desktop and ChatGPT.

## Current Behavior

```json
{
"filename": "screenshot.png",
"download_url": "http://localhost:7498/files/.../screenshot.png",
"hint": "Use curl -o to save the file"
}
```

**Requires:** Agent must run `curl` then read the file - only works with filesystem-capable agents.

## Proposed Behavior

Add `inline` parameter to `shell_screenshot`:

```typescript
inline: z.boolean().optional().describe("Return base64 image in response instead of download URL (default: false)")
```

When `inline: true`:
```json
{
"content": [
{ "type": "image", "data": "", "mimeType": "image/png" }
]
}
```

## Implementation Options

1. **Per-call parameter** - `inline: true` on screenshot calls
2. **Server config** - `--inline-images` flag
3. **Both** - config sets default, parameter overrides

## Testing Plan

### Before (current - should fail)

Test with Claude Desktop (MCP-only, no filesystem):

1. Configure Shellwright as MCP server in Claude Desktop
2. Ask: "Start a bash session, run `ls`, and show me a screenshot"
3. **Expected:** Claude receives URL but cannot fetch/display it
4. **Actual behavior to document:** What error or limitation does the user see?

### After (with inline mode)

Same test with `inline: true` or `--inline-images`:

1. Configure Shellwright with inline mode enabled
2. Ask: "Start a bash session, run `ls`, and show me a screenshot"
3. **Expected:** Screenshot appears inline in Claude Desktop conversation

### Test Matrix

| Client | Filesystem Access | Current | With Inline |
|--------|------------------|---------|-------------|
| Claude Code | ✅ | ✅ Works | ✅ Works |
| Claude Desktop | ❌ | ❌ URL only | ✅ Works |
| ChatGPT + MCP | ❌ | ❌ URL only | ✅ Works |
| Cursor | ✅ | ✅ Works | ✅ Works |

## Context Window Considerations

Inline images consume significant context (~50-200KB base64 per screenshot). Consider:

- Scaling images down (like Playwright MCP's 1.15MP / 1568px limit)
- Warning in docs about context usage
- Keeping URL mode as default for context-sensitive workflows

## References

- Playwright MCP's approach: Returns scaled base64 images inline
- `scaleImageToFitMessage()` limits to Claude's vision requirements (1.15 megapixels max)