Subagent isolation options adjust for sessions without a disk

What

A subagent is a helper agent that Claude starts to work on part of a task. It can be isolated from your main work in a separate git worktree (a second working copy of the repository) or in a remote sandbox. Some sessions are now launched with no disk ("diskless"), and isolation adapts to them:

• Worktree isolation: creating an agent worktree now fails with "worktree isolation is unavailable in a session without a disk". The check behind isolation: "worktree" now also requires a disk. That check also decides the parallel-agents hint in Claude's instructions and whether remote agents are allowed.
• Background task records no longer include an outputFile in diskless sessions.
• Remote isolation fallback: when isolation: "remote" is unavailable in a diskless session, the agent runs locally with the note "(the session has no disk); running as a local agent". Before, it fell back to a worktree whenever there was a git repository.
• Remote isolation eligibility: the check that decides whether the Agent tool offers isolation: "remote" now first excludes diskless sessions and adds a new requirement. These run before the existing checks. It still ends on the server-controlled flag tengu_neapolitan, whose built-in fallback is false.

The Agent tool's stock description captured under this release is unchanged from the previous release.

Why

In sessions without a disk, subagents no longer attempt worktrees that cannot work. They run locally instead of failing. Fewer sessions are offered remote subagent isolation.