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>

test/e2e-models.test.ts

@@ -50,7 +50,6 @@ describe("Perplexity model selection e2e", () => {
           {
             query: "Say exactly OK",
             model,
-            incognito: true,
           },
           token,
         );