refactor(beat): replace startup fmt.Fprintf with structured slog.Error

[IanEff]

Closes #192

Summary

Replaces bare fmt.Fprintf(stderr, ...) and fmt.Fprintln(stderr, ...) during startup/initialization across all four beats (clank, hiss, rattle, thump) and beat.Start with structured slog.Error calls.

Changes

• internal/beat:
  • Attached "beat" attribute to the default JSON slog logger upon startup.
  • Flag parsing errors now emit structured slog.Error("failed to parse flags", "err", err) to stdout before exiting.
  • Audited BrokerHooks and removed redundant "beat" attribute logs.
  • Added table-driven cross-beat test verifying structured startup logs across all beats.
• internal/clank:
  • Converted config/catalog/weights/limits/tracer/metrics/transport/broker startup errors to slog.Error.
  • Cleaned up redundant "beat", "clank" attributes.
  • Updated startup tests to assert structured JSON records on stdout.
• internal/hiss:
  • Converted config/policy/tracer/metrics/transport/broker startup errors to slog.Error.
  • Cleaned up redundant "beat", "hiss" attributes.
  • Updated startup tests to assert structured JSON records on stdout.
• internal/rattle:
  • Converted config/watch/query/sources/metrics/transport/broker startup errors to slog.Error.
  • Cleaned up redundant "beat", "rattle" attributes.
  • Updated startup tests to assert structured JSON records on stdout.
• internal/thump:
  • Converted config/catalog/tracer/metrics/executor/reversal/broker startup errors to slog.Error.
  • Cleaned up redundant "beat", "thump" attributes.
  • Updated startup tests to assert structured JSON records on stdout.

Verification

• go test -race ./... passed.
• task ci (including gofmt, go vet, golangci-lint, govulncheck, kubeconform, promtool, calipers validate, go test -race ./..., task build) passed clean.