refactor(pi): standalone Zulip gateway service — replaces pi extension

Complete rewrite of the pi Zulip extension as a standalone Node.js service
with direct harness API calls. No longer depends on pi's session — survives
pi shutdown and restart.

Changes:
- src/index.ts: Complete rewrite — standalone process, not a pi extension
  - Direct harness API calls (POST /v1/chat/completions) with conversation memory
  - Per-sender conversation history (up to 50 messages)
  - Placeholder->edit streaming for Zulip DMs
  - Typing indicators via Zulip API
  - Health endpoint on :9200
  - Exponential backoff reconnection
  - Graceful shutdown on SIGINT/SIGTERM
- abiba-zulip.service: systemd service unit file
- README.md: Updated deployment instructions
- package.json/tsconfig.json: Updated for standalone app

Deploy: npm install -> npx tsc -> systemctl enable abiba-zulip
Closes issue #24 (Hermes parity for pi)

pi-zulip-extension/package.json

@@ -1,28 +1,19 @@
 {
-  "name": "pi-zulip-extension",
-  "version": "0.1.0",
-  "description": "pi extension for Zulip agent communication — connects Abiba to the Sysloggh agent mesh",
-  "main": "src/index.ts",
+  "name": "abiba-zulip-service",
+  "version": "2.0.0",
+  "description": "Standalone Zulip gateway service for Abiba (pi agent). Direct harness API, conversation memory, systemd service.",
   "type": "module",
+  "main": "dist/index.js",
   "scripts": {
     "build": "tsc",
-    "check": "tsc --noEmit"
+    "start": "node dist/index.js",
+    "dev": "node --watch dist/index.js"
   },
   "dependencies": {
-    "yaml": "^2.9.0",
     "zulip-js": "^2.0.0"
   },
   "devDependencies": {
-    "@earendil-works/pi-coding-agent": "^0.80.2",
-    "typescript": "^5.0.0"
-  },
-  "keywords": [
-    "pi",
-    "zulip",
-    "agent",
-    "abiba",
-    "sysloggh"
-  ],
-  "license": "UNLICENSED",
-  "private": true
+    "@types/node": "^22.0.0",
+    "typescript": "^5.7.0"
+  }
 }