refactor(cli): polish output formatting

[koistya]

Summary

• Consistent activity sigils: + connected, - disconnected, → RPC, ← echo, ⚠ warning
• Short peer IDs (first 8 chars) for readability
• Dedicated printEcho/emitEcho with safe serialization (handles circular refs, BigInt, undefined)
• QC expiry message moved from stderr to stdout with timestamp
• Streamlined printFatal format

  + Connected (8f3kN2p1) [12:34:05]
  ← echo: hello [12:34:06]
  → $sideband/info [12:34:07]
  - Disconnected (8f3kN2p1) [12:34:12]

Test plan

• Existing output tests updated and passing
• New tests for emitEcho (data payload, circular refs, undefined normalization)
• printEcho safe stringify test for non-serializable data
• Manual: npx sideband and verify human-mode output formatting
• Manual: npx sideband --json and verify NDJSON echo events include data field