feat(memory): add parameterized compaction prompt builder

src/backends/native/prompts.ts

@@ -25,6 +25,45 @@ Rules:
 Output format:
 Return a markdown summary with bullet points. Do not include any preamble or explanation — output only the summary.`;
 
+/**
+ * Build a compaction system prompt. When `personalAssistant` is true,
+ * the prompt focuses on continuity context (what the user was working on,
+ * decisions, preferences, open threads) rather than generic summarization.
+ *
+ * The shared rules (preserve facts, 20% length, bullet points, no invention,
+ * skip transient content) are identical in both variants.
+ */
+export function buildCompactionPrompt(opts?: { personalAssistant?: boolean }): string {
+  const focus = opts?.personalAssistant
+    ? `Focus on:
+- What the user was working on and its current status (be specific: which files, commands, or steps were involved)
+- Decisions made and why (include rationale when stated)
+- Preferences or constraints the user expressed (tools, styles, approaches to avoid or prefer)
+- Open threads, unresolved questions, or explicit follow-up items`
+    : `Focus on:
+- Key topics discussed and conclusions reached
+- Important decisions, commitments, or action items
+- Technical details, code changes, or configurations that were established`;
+
+  const preamble = opts?.personalAssistant
+    ? 'You are summarising a conversation for a personal assistant. Your summary will be injected at the start of the next session so the assistant can pick up exactly where things left off.'
+    : 'You are a conversation summarizer. Create a concise summary of the conversation that captures all important information.';
+
+  return `${preamble}
+
+${focus}
+
+Rules:
+- Preserve key facts, file paths, error messages, and specific values verbatim.
+- Be concise but complete — aim for roughly 20% of the original length.
+- Use bullet points under short descriptive headings.
+- Never invent information not present in the conversation.
+- Skip purely transient content (one-off commands, status messages with no lasting significance).
+
+Output format:
+Return a markdown summary. No preamble — output only the summary.`;
+}
+
 /**
  * Instructs a model to extract persistent facts from conversation text.
  * Extracted facts are stored in long-term memory for future sessions.