Add override prompt variable (#301)

* Add override prompt variable

* create test

* Fix typechecks

* remove use of `any` for additional type-safety

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Co-authored-by: km-anthropic <km-anthropic@users.noreply.github.com>

test/install-mcp-server.test.ts

@@ -31,6 +31,7 @@ describe("prepareMcpConfig", () => {
       disallowedTools: [],
       customInstructions: "",
       directPrompt: "",
+      overridePrompt: "",
       branchPrefix: "",
       useStickyComment: false,
       additionalPermissions: new Map(),