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/ai/CHANGELOG.md

@@ -0,0 +1,15 @@
+# Changelog
+
+## [Unreleased]
+
+### Fixed
+
+- **Fixed Claude Opus 4.5 cache pricing** (was 3x too expensive)
+  - Corrected cache_read: $1.50 → $0.50 per MTok
+  - Corrected cache_write: $18.75 → $6.25 per MTok
+  - Added manual override in `scripts/generate-models.ts` until upstream fix is merged
+  - Submitted PR to models.dev: https://github.com/sst/models.dev/pull/439
+
+## [0.9.4] - 2025-11-26
+
+Initial release with multi-provider LLM support.