3747-3: EOS syslog + tail-based readers (~250 LOC code)

[nikw9944]

Parent issue: #3747

Split from plan review of #3747 (estimated ~250 lines).

PR stack: this issue's branch should target nikw9944/doublezero-3794 (the previous sub-issue's branch).

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• Creates internal/tailer/tailer.go (shared poll-based tailer with
  open + offset + recreate/truncate detection + partial-line buffering).
• Creates internal/loggingtail/eos.go (show logging last <window> poller
  with parse + dedupe + NDJSON output).
• Creates internal/loggingtail/agent.go (uses tailer.Tailer against
  <working-dir>/orchestrator.agent.log; emits typed events).
• Creates internal/runlog/reader.go (uses tailer.Tailer against
  <working-dir>/orchestrator-runlog.json; owns pending-submit maps and
  bounded duration rings; exposes ProvisionDurations(window) and
  DeprovisionDurations(window)).
• Updates README with EOS-log / agent-log / runlog sections.