mom: add centralized logging, usage tracking, and improve prompt caching

Major improvements to mom's logging and cost reporting:

Centralized Logging System:
- Add src/log.ts with type-safe logging functions
- Colored console output (green=user, yellow=mom, dim=details)
- Consistent format: [HH:MM:SS] [context] message
- Replace scattered console.log/error calls throughout codebase

Usage Tracking & Cost Reporting:
- Track tokens (input, output, cache read/write) and costs per run
- Display summary at end of each run in console and Slack thread
- Example: 💰 Usage: 12,543 in + 847 out (5,234 cache read) = $0.0234

Prompt Caching Optimization:
- Move recent messages from system prompt to user message
- System prompt now mostly static (only changes with memory files)
- Enables effective use of Anthropic's prompt caching
- Significantly reduces costs on subsequent requests

Model & Cost Improvements:
- Switch from Claude Opus 4.5 to Sonnet 4.5 (~40% cost reduction)
- Fix Claude Opus 4.5 cache pricing in ai package (was 3x too expensive)
- Add manual override in generate-models.ts until upstream fix merges
- Submitted PR to models.dev: https://github.com/sst/models.dev/pull/439

UI/UX Improvements:
- Extract actual text from tool results instead of JSON wrapper
- Cleaner Slack thread formatting with duration and labels
- Tool args formatting shows paths with offset:limit notation
- Add chalk for colored terminal output

Dependencies:
- Add chalk package for terminal colors

packages/mom/src/store.ts

@@ -1,6 +1,7 @@
 import { existsSync, mkdirSync } from "fs";
 import { appendFile, writeFile } from "fs/promises";
 import { join } from "path";
+import * as log from "./log.js";
 
 export interface Attachment {
 	original: string; // original filename from uploader
@@ -152,10 +153,10 @@ export class ChannelStore {
 
 			try {
 				await this.downloadAttachment(item.localPath, item.url);
-				console.log(`Downloaded: ${item.localPath}`);
+				// Success - could add success logging here if we have context
 			} catch (error) {
-				console.error(`Failed to download ${item.localPath}:`, error);
-				// Could re-queue for retry here
+				const errorMsg = error instanceof Error ? error.message : String(error);
+				log.logWarning(`Failed to download attachment`, `${item.localPath}: ${errorMsg}`);
 			}
 		}