fix(coding-agent): string systemPrompt now works as full replacement

When passing a string systemPrompt to createAgentSession(), it is now
used as-is without appending context files and skills. This matches the
documented behavior: 'String replaces default, function receives default
and returns final.'

Previously, string systemPrompt would have context files and skills
appended, causing duplication if they were already in the string.

fixes #543

packages/coding-agent/CHANGELOG.md

@@ -17,6 +17,7 @@
 
 ### Fixed
 
+- String `systemPrompt` in `createAgentSession()` now works as a full replacement instead of having context files and skills appended, matching documented behavior ([#543](https://github.com/badlogic/pi-mono/issues/543))
 - Update notification for bun binary installs now shows release download URL instead of npm command ([#567](https://github.com/badlogic/pi-mono/pull/567) by [@ferologics](https://github.com/ferologics))
 - ESC key now works during "Working..." state after auto-retry ([#568](https://github.com/badlogic/pi-mono/pull/568) by [@tmustier](https://github.com/tmustier))
 - Abort messages now show correct retry attempt count (e.g., "Aborted after 2 retry attempts") ([#568](https://github.com/badlogic/pi-mono/pull/568) by [@tmustier](https://github.com/tmustier))

packages/coding-agent/src/core/sdk.ts

@@ -549,14 +549,8 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		if (options.systemPrompt === undefined) {
 			return defaultPrompt;
 		} else if (typeof options.systemPrompt === "string") {
-			return buildSystemPromptInternal({
-				cwd,
-				agentDir,
-				skills,
-				contextFiles,
-				selectedTools: validToolNames,
-				customPrompt: options.systemPrompt,
-			});
+			// String is a full replacement - use as-is without appending context/skills
+			return options.systemPrompt;
 		} else {
 			return options.systemPrompt(defaultPrompt);
 		}