feat: supporto ai Focus Modes di Perplexity (parametro 'focus')

- Aggiunto il parametro 'focus' al tool perplexity_search per selezionare la
  fonte di conoscenza (Focus Mode).
- FOCUS_MAP: mappa focus -> search_focus + sources usati dall'endpoint
  perplexity_ask (internet/web, academic/scholar, writing, wolfram, youtube,
  reddit, math, social, finance).
- default: internet (comportamento invariato).
- README aggiornato.

README.md

@@ -74,8 +74,12 @@ Once installed, the agent automatically calls `perplexity_search` whenever it ne
 |---|---|---|---|
 | `query` | string | ✅ | The search query |
 | `recency` | string | — | Filter by age: `hour` · `day` · `week` · `month` · `year` |
+| `focus` | string | — | Perplexity **Focus Mode** (source domain): `internet` · `academic` · `writing` · `wolfram` · `youtube` · `reddit` · `math` · `social` · `finance` (default `internet`) |
 | `limit` | number | — | Max sources to include (1–50) |
 
+**Focus Modes** let the agent route the search to a specific knowledge source:
+`academic` → peer-reviewed/scholarly (PubMed, arXiv, Semantic Scholar), `youtube` → video transcripts, `reddit` → community/opinions, `wolfram`/`math` → computations, `social` → social platforms, `finance` → financial/SEC data, `writing` → model-only (no live search).
+
 Model selection is configured globally with `/perplexity-config` or `PI_PERPLEXITY_MODEL`; it is not exposed as a tool parameter, so agent-generated tool calls cannot accidentally override your configured model.
 
 ### Output format