Add Custom Metrics: user-supplied JSON sources shown in the panel menu

[surendra10125]

Adds a new "Custom" sensor category, alongside Temperature/Network/etc., that reads JSON files (title, metricsBarValue, metrics[]) configured via Preferences and displays each metric as a row, with any row (including a per-source summary) pinnable to the top bar like any other sensor.

Includes custom_metrics/claude-code-usage/, a companion script that feeds Claude Code's own statusLine data (model, context %, 5h/7d rate-limit usage) into a custom-metrics JSON source, with a self-installing login item so it stays wired up across logins.

[surendra10125]

@corecoding please have look into this

[surendra10125]

@corecoding can you please look into this

[corecoding]

I like the idea of adding custom items to the menu, but having bash scripts, python code, etc will trigger a red flag with the gnome extension review team. Also puts ownership on me to support and maintain these Claude companion scripts going forward. I'd rather just read from a custom file in /tmp called vitals.json and if it exists, it'll unpack and display the values. The supporting Claude scripts can live outside vitals.