refactor(search): always send searches as incognito (#14)

Hardcode is_incognito: true in the Perplexity request and remove the
toggle surface: the incognito tool parameter, PI_PERPLEXITY_INCOGNITO
env var, config file field, /perplexity-config prompt, and TUI status
indicators. Config and resolveDefaultModel are now model-only.

Supersedes the incognito portion of PR #4.

Co-authored-by: Ivan Pereira <183991+ivanrvpereira@users.noreply.github.com>

README.md

@@ -75,7 +75,6 @@ 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` |
 | `limit` | number | — | Max sources to include (1–50) |
-| `incognito` | boolean | — | Whether to hide the search from Perplexity history; defaults to `true` |
 
 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.
 
@@ -102,7 +101,7 @@ Provider: perplexity (oauth)
 Model: pplx_pro_upgraded
 ```
 
-Queries default to `is_incognito: true`, but you can override that per call or via config.
+Queries always send `is_incognito: true`, so searches never appear in your Perplexity web history.
 
 ## How It Works