fix: block memory agent prose-skip responses at prompt and runtime levels

Observer prompt now explicitly requires XML observation blocks or empty
responses — prose explanations like "Skipping" are discarded. ResponseProcessor
logs a warning when non-XML content is received. Recording focus expanded to
include concrete debugging findings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/sdk/prompts.ts

@@ -116,7 +116,11 @@ export function buildObservationPrompt(obs: Observation): string {
   <occurred_at>${new Date(obs.created_at_epoch).toISOString()}</occurred_at>${obs.cwd ? `\n  <working_directory>${obs.cwd}</working_directory>` : ''}
   <parameters>${JSON.stringify(toolInput, null, 2)}</parameters>
   <outcome>${JSON.stringify(toolOutput, null, 2)}</outcome>
-</observed_from_primary_session>`;
+</observed_from_primary_session>
+
+Return either one or more <observation>...</observation> blocks, or an empty response if this tool use should be skipped.
+Concrete debugging findings from logs, queue state, database rows, session routing, or code-path inspection count as durable discoveries and should be recorded.
+Never reply with prose such as "Skipping", "No substantive tool executions", or any explanation outside XML. Non-XML text is discarded.`;
 }
 
 /**
@@ -235,4 +239,4 @@ ${mode.prompts.format_examples}
 ${mode.prompts.footer}
 
 ${mode.prompts.header_memory_continued}`;
-} 
+}