feat: `--watch` flag for `meshd status --json` (live streaming JSON)

[gitcoder89431]

Summary

Add a --watch flag to meshd status --json that streams newline-delimited JSON on state change instead of a one-shot output.

Motivation

The current --json flag works for one-shot scripts but Quickshell widgets need to poll on a timer, which is janky and wastes CPU. A --watch mode would make widgets fully reactive with zero overhead — they just read stdin.

Proposed Behavior

meshd status --json --watch
# streams a new JSON line every time state changes (device connects, file syncs, conflict detected, etc.)
# powered by /rest/events?since=<n> under the hood

Output format (newline-delimited JSON):

{"devices":[{"name":"MacBook","status":"online","completion":100}],"syncing":false,"conflicts":0}
{"devices":[{"name":"MacBook","status":"syncing","completion":73}],"syncing":true,"conflicts":0}

Implementation Notes

• Subscribe to /rest/events?since=<n> (long-poll, already used by the TUI)
• On each relevant event, re-fetch /rest/db/status and emit a new JSON line to stdout
• Relevant event types: StateChanged, DeviceConnected, DeviceDisconnected, FolderCompletion, LocalChangeDetected
• Exits cleanly on ctrl+c / SIGTERM

Use Cases

• Quickshell / EWW / Waybar widgets that show sync status in a bar
• Shell scripts that wait for sync to complete before proceeding
• Agent workflows polling for file propagation confirmation