feat(miner): Add JSON output mode to clawrtc mine

[Tianlin0725]

Summary

Add --json flag to the universal miner that makes the miner output structured JSON instead of human-readable text. This lets other tools and scripts parse miner output programmatically.

Changes

• Add --json and --dry-run flags to argparse
• Create emit() helper method that outputs either JSON (JSONL format) or human-readable text based on json_mode flag
• Update attestation and mining loop to use emit() instead of print()

Expected Behavior

# Normal output (unchanged)
$ clawrtc mine --wallet test
RustChain Universal Miner v2.3.0
Miner ID:    ...
Wallet:      ...
[12:34:56] Attestation: success

# JSON output (new)
$ clawrtc mine --wallet test --json --dry-run
{"event":"startup","timestamp":1234567890,"wallet":"test","node":"https://50.28.86.131","hardware":{"arch":"x86_64","family":"modern"}}
{"event":"ready","status":"dry-run complete"}

Related Issue

Closes #346 (GOOD FIRST ISSUE)