Enhancement: Return pagination metadata in semantic_code_search response

## Summary

Add `totalHits` to the `semantic_code_search` response to help agents make informed decisions about whether to paginate further or switch to other tools like `map_symbols_by_query`.

## Current Behavior

The tool accepts `page` and `size` parameters but returns only the hits array:

```typescript
// src/mcp_server/tools/semantic_code_search.ts:84-101
return {
content: [
{
type: 'text',
text: JSON.stringify(
response.hits.hits.map((hit) => {
const { type, language, kind, filePath, content } = hit._source;
return { score: hit._score, type, language, kind, filePath, content };
})
),
},
],
};
```

Response shape:
```json
[
{ "score": 0.92, "type": "code", "filePath": "...", "content": "..." },
{ "score": 0.87, "type": "code", "filePath": "...", "content": "..." }
]
```

## Proposed Behavior

Include pagination metadata from the already-available `response.hits.total`:

```json
{
"hits": [
{ "score": 0.92, "type": "code", "filePath": "...", "content": "..." },
{ "score": 0.87, "type": "code", "filePath": "...", "content": "..." }
],
"total": 142
}
```

## Why This Helps Agents

```
┌────────────────────────────────────────────────────────────────────┐
│ Agent receives: { hits: [...25 items], total: 142 } │
├────────────────────────────────────────────────────────────────────┤
│ │
│ Agent can now reason: │
│ │
│ • "142 total results, I have 25 - there's more if I need it" │
│ • "142 is a lot - maybe map_symbols_by_query is better for │
│ exhaustive coverage" │
│ • "Only 8 total results - no need to paginate" │
│ │
└────────────────────────────────────────────────────────────────────┘
```

## Implementation

Minimal change in `src/mcp_server/tools/semantic_code_search.ts`:

```typescript
// Extract total from ES response (already available)
const total = typeof response.hits.total === 'number'
? response.hits.total
: response.hits.total?.value ?? 0;

return {
content: [
{
type: 'text',
text: JSON.stringify({
hits: response.hits.hits.map((hit) => {
const { type, language, kind, filePath, content } = hit._source;
return { score: hit._score, type, language, kind, filePath, content };
}),
total,
}),
},
],
};
```

## Acceptance Criteria

- [ ] Response includes `total` field with the total number of matching documents
- [ ] Existing `hits` array structure unchanged (backward compatible for agents parsing the array)