feat: massive overhaul to manage costs (#103)

### Explanation

_No response_

### Issue

Closes #102

### Attestations

- [ ] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
- [ ] I have read and agree to the [Community Guidelines](https://docs.nhcarrigan.com/community/guide/).
- [ ] My contribution complies with the [Contributor Covenant](https://docs.nhcarrigan.com/dev/covenant/).

### Dependencies

- [ ] I have pinned the dependencies to a specific patch version.

### Style

- [ ] I have run the linter and resolved any errors.
- [ ] My pull request uses an appropriate title, matching the conventional commit standards.
- [ ] My scope of feat/fix/chore/etc. correctly matches the nature of changes in my pull request.

### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
- [ ] All new and existing tests pass locally with my changes.
- [ ] Code coverage remains at or above the configured threshold.

### Documentation

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### Versioning

_No response_

Reviewed-on: #103
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/lib/stores/conversations.ts

@@ -11,6 +11,13 @@ import { cleanupConversationTracking } from "$lib/tauri";
 import { characterState } from "$lib/stores/character";
 import { sessionsStore } from "$lib/stores/sessions";
 
+export interface ConversationSummary {
+  generatedAt: Date;
+  content: string;
+  messageCount: number;
+  tokenEstimate: number;
+}
+
 export interface Conversation {
   id: string;
   name: string;
@@ -27,6 +34,7 @@ export interface Conversation {
   createdAt: Date;
   lastActivityAt: Date;
   attachments: Attachment[];
+  summary: ConversationSummary | null;
 }
 
 function createConversationsStore() {
@@ -63,6 +71,7 @@ function createConversationsStore() {
       createdAt: new Date(),
       lastActivityAt: new Date(),
       attachments: [],
+      summary: null,
     };
   }
 
@@ -420,7 +429,12 @@ function createConversationsStore() {
       });
     },
 
-    addLine: (type: TerminalLine["type"], content: string, toolName?: string) => {
+    addLine: (
+      type: TerminalLine["type"],
+      content: string,
+      toolName?: string,
+      cost?: TerminalLine["cost"]
+    ) => {
       ensureInitialized();
       const activeId = get(activeConversationId);
       if (!activeId) return "";
@@ -431,6 +445,7 @@ function createConversationsStore() {
         content,
         timestamp: new Date(),
         toolName,
+        cost,
       };
 
       conversations.update((convs) => {
@@ -451,7 +466,8 @@ function createConversationsStore() {
       conversationId: string,
       type: TerminalLine["type"],
       content: string,
-      toolName?: string
+      toolName?: string,
+      cost?: TerminalLine["cost"]
     ) => {
       ensureInitialized();
 
@@ -461,6 +477,7 @@ function createConversationsStore() {
         content,
         timestamp: new Date(),
         toolName,
+        cost,
       };
 
       conversations.update((convs) => {
@@ -636,6 +653,130 @@ function createConversationsStore() {
       return conv?.attachments || [];
     },
 
+    // Summary/compaction functions
+    setSummary: (conversationId: string, summary: ConversationSummary) => {
+      conversations.update((convs) => {
+        const conv = convs.get(conversationId);
+        if (conv) {
+          conv.summary = summary;
+          conv.lastActivityAt = new Date();
+        }
+        return convs;
+      });
+    },
+
+    clearSummary: (conversationId: string) => {
+      conversations.update((convs) => {
+        const conv = convs.get(conversationId);
+        if (conv) {
+          conv.summary = null;
+          conv.lastActivityAt = new Date();
+        }
+        return convs;
+      });
+    },
+
+    getSummary: (conversationId: string): ConversationSummary | null => {
+      const convs = get(conversations);
+      const conv = convs.get(conversationId);
+      return conv?.summary || null;
+    },
+
+    // Estimate token count for a conversation (rough approximation: ~4 chars per token)
+    estimateTokenCount: (conversationId: string): number => {
+      const convs = get(conversations);
+      const conv = convs.get(conversationId);
+      if (!conv) return 0;
+
+      const relevantLines = conv.terminalLines.filter(
+        (line) => line.type === "user" || line.type === "assistant"
+      );
+
+      const totalChars = relevantLines.reduce((sum, line) => sum + line.content.length, 0);
+      return Math.ceil(totalChars / 4);
+    },
+
+    // Get conversation content suitable for summarisation
+    getConversationForSummary: (conversationId: string): string => {
+      const convs = get(conversations);
+      const conv = convs.get(conversationId);
+      if (!conv) return "";
+
+      const relevantLines = conv.terminalLines.filter(
+        (line) => line.type === "user" || line.type === "assistant"
+      );
+
+      return relevantLines
+        .map((line) => {
+          const role = line.type === "user" ? "User" : "Assistant";
+          return `${role}: ${line.content}`;
+        })
+        .join("\n\n");
+    },
+
+    // Compact conversation by keeping only recent messages
+    compactConversation: (conversationId: string, keepRecentCount: number = 10) => {
+      conversations.update((convs) => {
+        const conv = convs.get(conversationId);
+        if (conv && conv.terminalLines.length > keepRecentCount) {
+          // Keep system messages and the most recent user/assistant messages
+          const systemLines = conv.terminalLines.filter(
+            (line) => line.type !== "user" && line.type !== "assistant"
+          );
+          const chatLines = conv.terminalLines.filter(
+            (line) => line.type === "user" || line.type === "assistant"
+          );
+
+          // Keep only the most recent chat messages
+          const recentChatLines = chatLines.slice(-keepRecentCount);
+
+          // Combine: system lines at original positions + recent chat lines
+          conv.terminalLines = [...systemLines.slice(-5), ...recentChatLines];
+          conv.lastActivityAt = new Date();
+        }
+        return convs;
+      });
+    },
+
+    // Compact conversation with a summary - clears old messages and injects summary context
+    compactWithSummary: (
+      conversationId: string,
+      summaryContent: string,
+      messageCount: number,
+      tokenEstimate: number
+    ) => {
+      conversations.update((convs) => {
+        const conv = convs.get(conversationId);
+        if (conv) {
+          // Store the summary
+          conv.summary = {
+            generatedAt: new Date(),
+            content: summaryContent,
+            messageCount,
+            tokenEstimate,
+          };
+
+          // Clear all messages and add a context injection message
+          conv.terminalLines = [
+            {
+              id: generateLineId(),
+              type: "system",
+              content: `[Conversation compacted] Previous session had ${messageCount} messages (~${tokenEstimate.toLocaleString()} tokens). Context preserved below.`,
+              timestamp: new Date(),
+            },
+            {
+              id: generateLineId(),
+              type: "system",
+              content: `Previous Session Context:\n${summaryContent}`,
+              timestamp: new Date(),
+            },
+          ];
+          conv.lastActivityAt = new Date();
+        }
+        return convs;
+      });
+    },
+
     // Add initialization helper
     initialize: () => {
       ensureInitialized();