refactor: implement configurable directory structure with XDG support

Separate configuration from data storage and make paths configurable
via environment variables and command-line flags. This improves
Unix/Linux compliance and supports both development and production
deployments.

Key changes:
- Separate config dir (opal.yml) from data dir (database, logs)
- Support XDG Base Directory specification
- Add --config-dir and --data-dir flags
- Environment variables: OPAL_CONFIG_DIR, OPAL_DATA_DIR, OPAL_DB_PATH
- Smart fallback: /etc/opal, /var/lib/opal -> ~/.config/opal, ~/.local/share/opal
- Server mode validates required OAuth/JWT environment variables
- Update naming from 'jade' to 'opal' throughout
- Update systemd service name to 'opal.service'
- Add migration guide in README

Default paths:
- Config: /etc/opal (fallback: ~/.config/opal)
- Data: /var/lib/opal (fallback: ~/.local/share/opal)

Files modified:
- internal/engine/config.go: New directory resolution logic
- internal/engine/database.go: Auto-create data directory
- cmd/root.go: Add global flags for directory overrides
- cmd/server.go: Add configuration validation
- cmd/sync.go, internal/sync/*: Use new path helper functions
- tests: Update to use directory overrides
- docs: Update deployment guide and README

opal-task/README.md

@@ -32,7 +32,56 @@ A task can be recurring. Then we have a template task and instances of that task
 A recurring task is given a status of recurring which hides it from view. The recurring task you create is called the template task, from which recurring tasks instances are created. So the template remains hidden, and the recurring instances that spawn from it are the tasks that you will see and complete.
 
 ## Storage
-SQLite database stored in `~/.config/jade/opal.db`
+
+**Configuration:** `~/.config/opal/opal.yml` (or `$XDG_CONFIG_HOME/opal/opal.yml`)  
+**Database:** `~/.local/share/opal/opal.db` (or `$XDG_DATA_HOME/opal/opal.db`)
+
+### Customizing Storage Locations
+
+Override default locations using environment variables or command-line flags:
+
+**Environment Variables:**
+- `OPAL_CONFIG_DIR` - Override config directory location
+- `OPAL_DATA_DIR` - Override data directory location  
+- `OPAL_DB_PATH` - Override database file path specifically
+- `XDG_CONFIG_HOME` - Standard XDG config directory (defaults to `~/.config`)
+- `XDG_DATA_HOME` - Standard XDG data directory (defaults to `~/.local/share`)
+
+**Command-Line Flags:**
+```bash
+opal --config-dir /custom/config --data-dir /custom/data list
+```
+
+**Production Server Setup:**
+```bash
+# Use system-wide paths for production
+OPAL_CONFIG_DIR=/etc/opal OPAL_DATA_DIR=/var/lib/opal opal server start
+```
+
+### Migrating from Old Paths
+
+If you previously used `~/.config/jade/`, you can migrate your data:
+
+```bash
+# Backup first (recommended)
+cp -r ~/.config/jade ~/.config/jade.backup
+
+# Create new directories
+mkdir -p ~/.config/opal ~/.local/share/opal
+
+# Copy config
+cp ~/.config/jade/opal.yml ~/.config/opal/
+
+# Move database and sync data
+mv ~/.config/jade/opal.db ~/.local/share/opal/
+mv ~/.config/jade/sync_*.* ~/.local/share/opal/ 2>/dev/null || true
+
+# Test that everything works
+opal list
+
+# Remove old directory after confirming it works
+rm -rf ~/.config/jade
+```
 
 ## Server & Sync