Task 2: Add GetPeerHandshakes() method to WgManager interface + impl + stub

Changes:
1. manager.go: Added PeerHandshake struct (PublicKey, LastHandshakeTime, RxBytes, TxBytes) + GetPeerHandshakes() ([]PeerHandshake, error) to WgManager interface
2. wgmanager_linux.go: Implemented on LinuxWgManager — opens wgctrl, reads device peers, returns []PeerHandshake. Uses mu.Lock/Unlock. Returns nil, nil on error.
3. wgmanager_stub.go: Implemented on StubWgManager — returns nil, nil

Verification:
- go build -tags dev ./... — PASSES
- go build ./... — PASSES
- lsp_diagnostics — CLEAN (1 pre-existing hint unrelated)