Probe v2.5.0 - Intel GPU Support

MINOR release: New feature added

**New Features:**
- Intel GPU monitoring via sysfs
- Support for 60+ Intel GPU models (Haswell to Raptor Lake + Arc)
- Clock frequency monitoring
- Power management (RC6) monitoring

**Supported Intel GPUs:**
- Integrated: HD Graphics 4600, UHD Graphics 770, Iris Xe, etc.
- Discrete: Arc A310, A380, A580, A750, A770

**Available Metrics:**
- Current/Min/Max/Boost clock frequencies
- RP0/RP1/RPn frequency levels
- RC6 power state residency

**Limitations:**
- No GPU utilization (not exposed by most Intel GPUs)
- No temperature on older generations
- No memory usage (integrated GPUs share system RAM)

**Files Changed:**
- internal/gpu/intel.go: New Intel GPU implementation
- internal/gpu/gpu.go: Multi-vendor integration
- probe.go: Version bump to 2.5.0