Initial postgres skill implementation

SKILL.md

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+---
+name: postgres
+description: PostgreSQL database server for persistent relational storage
+metadata:
+  version: "1.0.0"
+  vibestack:
+    main: false
+    metrics-port: 9187
+---
+
+# PostgreSQL Skill
+
+Installs and configures [PostgreSQL](https://www.postgresql.org/) database server for persistent relational data storage.
+
+## Features
+
+- Persistent relational database
+- Auto-creates database and user on first run
+- WAL-based replication ready
+- Connection string exposed as `DATABASE_URL`
+- Optional metrics via postgres_exporter
+
+## Configuration
+
+### Environment Variables
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `POSTGRES_PORT` | `5432` | PostgreSQL port |
+| `POSTGRES_USER` | `vibestack` | Default user |
+| `POSTGRES_PASSWORD` | `vibestack` | Default password |
+| `POSTGRES_DB` | `vibestack` | Default database |
+| `POSTGRES_DATA_DIR` | `/data/postgres` | Data directory |
+| `POSTGRES_INITDB_ARGS` | (none) | Extra initdb arguments |
+| `POSTGRES_MAX_CONNECTIONS` | `100` | Maximum connections |
+
+### Output Variables
+
+After starting, the skill exports:
+
+| Variable | Example |
+|----------|---------|
+| `DATABASE_URL` | `postgresql://vibestack:vibestack@localhost:5432/vibestack` |
+
+## Usage
+
+### As a dependency
+
+```yaml
+metadata:
+  vibestack:
+    requires:
+      - postgres
+```
+
+### Connection
+
+```bash
+# Connect with psql
+psql "$DATABASE_URL"
+
+# From application
+DATABASE_URL=postgresql://vibestack:vibestack@localhost:5432/vibestack
+```
+
+### Custom initialization
+
+Place SQL files in `initdb.d/` directory - they'll be executed on first startup:
+
+```
+skills/postgres/
+├── initdb.d/
+│   ├── 01-create-tables.sql
+│   └── 02-seed-data.sql
+```
+
+## Data Persistence
+
+All data is stored in `POSTGRES_DATA_DIR` (default: `/data/postgres`). Mount this directory to persist data across container restarts.
+
+## Backup Integration
+
+The `backup` skill automatically backs up PostgreSQL data using pg_dump for consistent snapshots.
+
+## Security
+
+For production use:
+1. Set a strong `POSTGRES_PASSWORD`
+2. Restrict network access to trusted hosts
+3. Use SSL for connections (configure in postgresql.conf)