Implement Android CPU and memory sampling in perf payload #126

Follow-up to #101.

Goal

Populate real cpu and memory metrics for Android targets in agent-device perf --json / metrics --json.

Scope

• Android emulator + physical device
• Session-scoped perf command response
• Sampling metadata (method, unit, measuredAt)

Proposed data sources

• CPU: adb shell dumpsys cpuinfo (package-focused parsing)
• Memory: adb shell dumpsys meminfo <package>

Acceptance criteria

• metrics.cpu.available=true on Android when app/package context is available
• metrics.memory.available=true on Android when app/package context is available
• Payload includes sampling metadata and clear units
• Errors are normalized (code, message, hint, etc.) when sampling fails
• Unit tests added for parser/extraction and handler response shape
• README + website docs updated with platform support and caveats

Out of scope

• iOS CPU/memory
• FPS sampling