Background task indicator stuck (background_outstanding never decrements) after cancelling a delegated async agent task

[Charlie-1909]

Environment: Codeg v0.24.0 (latest at time of report), Windows 11

Diagnosed by: this report was investigated and written by Claude Code, running as the active agent inside a Codeg session, at the request of the human user. Claude Code inspected the app's own logs and running processes to trace the root cause below (not a guess from the UI alone).

Steps to reproduce:

1. In a Claude Code session running inside Codeg, delegate work to a sub-agent via the Task/Agent tool (this gets registered by Codeg as an "async agent task" under bg-watch).
2. Cancel/stop that sub-agent before it finishes on its own (e.g. by sending it a message telling it to stop).
3. The session's UI keeps showing "1 task running in background" indefinitely, even long after the sub-agent's connection has gone Connected -> Disconnected.

Evidence from ~/.codeg/logs/codeg.<date>.log:

• [bg-watch] registered async agent task=<task_id> is logged once when the agent is launched, incrementing the parent session's outstanding count to 1.
• No corresponding deregistration log line ever appears afterward, even after the task is cancelled and its own connection later logs Connected -> Disconnected.
• Every subsequent turn_complete event for the parent session logs background_outstanding=1, indefinitely (checked over many turns spanning ~50 minutes).
• Closing the window and reopening the app does not fix it — the underlying codeg.exe process never actually exits (same PID and StartTime before/after "restart"), so the in-memory counter survives. Only fully killing the process and relaunching resets the indicator.

Relation to other issues: This looks related to but distinct from #200 (closed by the maintainer as fixed in a newer version). That issue was about a normally-completed task not ending the session. This one is specifically triggered by cancelling an in-flight background agent task — the registration/deregistration pair for that specific code path appears to still be unbalanced.

Workaround: Fully kill the codeg.exe process (not just close the window) and relaunch.