Add fingerprint module for monthly chunk deduplication

New module: src/fingerprint/
- types.ts: MonthlyChunk, DeduplicationPlan, FingerprintConfig
- chunker.ts: generateMonthlyChunks(), createDeduplicationPlan()
- index.ts: Module exports

Key features:
- Groups messages by calendar month for deduplication
- Uses first 10 messages + message count per month for fingerprinting
- Truncates timestamps to minute (floor, not round) to handle WhatsApp ±1s drift
- Per-user isolation (fingerprints not shared across users)
- Includes comprehensive tests for edge cases including real-world drift scenario

See project_docs/CHAT_FINGERPRINTING.md for algorithm details.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ndbroadbent

package.json

@@ -37,6 +37,11 @@
       "types": "./dist/index.d.ts",
       "import": "./dist/index.js",
       "default": "./dist/index.js"
+    },
+    "./costs": {
+      "types": "./dist/costs/index.d.ts",
+      "import": "./dist/costs/index.js",
+      "default": "./dist/costs/index.js"
     }
   },
   "files": [
@@ -47,7 +52,7 @@
   "scripts": {
     "cli": "bun run src/cli.ts",
     "dev": "bun run --watch src/cli.ts",
-    "build": "bun build src/index.ts --outdir dist --target node && bun build src/cli.ts --outdir dist --target node && tsc -p tsconfig.types.json",
+    "build": "bun build src/index.ts --outdir dist --target node && bun build src/costs/index.ts --outdir dist/costs --target node && bun build src/cli.ts --outdir dist --target node && tsc -p tsconfig.types.json",
     "build:types": "tsc -p tsconfig.types.json",
     "build:types:watch": "tsc -p tsconfig.types.json --watch",
     "build:binary": "bun build src/cli.ts --compile --outfile chat-to-map",