Subagent history browser for finished subagents, behind CLAUDE_CODE_SUBAGENT_HISTORY

What

A subagent is a helper agent that Claude starts to work on part of a task. Claude Code now has code to list subagents that have already finished in the current session, so you can look back at them.

• The history loader reads each subagent's transcript (its recorded conversation) and its .meta.json sidecar file, which sits next to the transcript. It builds a parent and child tree from them and skips observer and taskKind entries.
• Each row shows a label, an @name or the agent type, a "started …" time, a "last active …" time and a "stopped" marker. The group is labelled "Earlier in this session".
• The Background dialog, which lists background tasks, builds a row of type subagent_history. It shows a count, or "could not be read" when the history cannot be read. Selecting it expands into recorded_agent rows for completed subagents, using stoppedByUser and lastActivityAt from the live task when there is one.
• The dialog lists the history even when no tasks are running. "No tasks currently running" now appears only when history rows are the only rows.
• A source-availability check that already returned false for monitor_ws and self_managed_tasks now also returns false for subagent_history and recorded_agent. Those two kinds are handled elsewhere in the same part of the code.
• The dream task detail view also gains a screen variant controlled by isBackgroundDetailScreenEnabled.

The feature reports itself as "off" unless its switch is true. An environment variable is a named value set in your shell before starting a program. The switch is the environment variable CLAUDE_CODE_SUBAGENT_HISTORY. When that variable is not set, Claude Code falls back to the server-side flag tengu_rustling_crescent, whose built-in default in the code is false. Nothing has been read about that flag's actual value.

Why

Before this, finished subagents could not be reviewed from this dialog. This change lets you look back at what they did and when they stopped. Setting CLAUDE_CODE_SUBAGENT_HISTORY is the switch the code checks first.