Fix Graphite panic on UDP dial failure

[jdevera]

Summary

• replace the archived graphite-golang client with the existing safe UDP sending path
• preserve the documented Graphite plaintext format and one-datagram-per-metric behavior
• return and log transport errors without changing command output or exit status
• remove the unused Graphite dependency
• add UDP wire-format and dial-failure regression tests

Sandbox behavior

When the launcher runs where UDP socket creation is denied, Graphite delivery now returns an ordinary operation-not-permitted error. The command keeps its stdout and original exit code. The failure is silent by default and is written to the launcher log when error logging is enabled.

Validation

• go test ./...
• go test ./cmd/metrics -count=10
• go vet ./...
• go mod tidy -diff
• reproduced UDP denial in a restricted sandbox and confirmed a successful launcher command still exits 0

Closes #12