feat: add prometheus plugin for metrics scraping

[418teapotcat]

Add Prometheus Plugin

New builtin plugin type for scraping Prometheus metrics and publishing them as variables on the launchpi device.

Features

• Configurable target URL - Connect to any Prometheus-compatible endpoint
• Scrape interval - Configurable polling (default 30s, minimum 10s)
• Bearer token auth - Optional authentication support
• Automatic variable publishing - Each scraped metric becomes a variable
• query action - Run arbitrary PromQL queries and publish results
• Preset readouts - CPU, memory, and disk usage with bar charts

Plugin Config Schema

[plugins.prometheus.monitoring]
target = "http://prometheus:9090"
scrape_interval_s = 30
bearer_token = "your-token-here"

Files Added

• daemon/src/plugins/builtin/prometheus/config.rs - Config structs
• daemon/src/plugins/builtin/prometheus/mod.rs - Plugin logic, scrape loop, presets
• daemon/src/plugins/builtin/mod.rs - Module registration

Notes

• Uses existing plugin architecture (manifest, config, presets, variables)
• Follows the same patterns as system and http plugins
• Metrics are truncated to 512 chars in the metrics variable (same as http plugin)
• Requires Prometheus to be reachable from the launchpi device