Task system session title leaks into main session (should be subagent/bg-task only)

Summary

The blue session title label in the CLI statusline (e.g. subagent-foobar-name) appears in the main interactive session even when the task was created as part of a subagent or background task workflow. Users expect this label to only appear for subagents and background tasks — not the primary session.

Steps to Reproduce

1. Run a workflow that spawns subagents (e.g. via Agent tool with subagent_type=Explore or similar)
2. Subagent creates or completes a task with a descriptive name
3. Main session statusline picks up the task name as its session title
4. Title persists in the main session even after subagent completes

Expected Behavior

• Session title in statusline should reflect the main session context (blank, or user-defined)
• Subagent/background task names should only appear in their own task context
• Main session title should not be overwritten by task names from child agents

Actual Behavior

Main session shows a task-derived title (e.g. subagent-foobar-name) that belongs to a completed subagent. The only way to clear it is /clear or closing/reopening the session.

Environment

• Claude Code CLI
• Task tools: TaskCreate / TaskUpdate used in subagent workflows
• Statusline enabled

Suggested Fix / Enhancement

• Scope task-derived session titles to the agent context that owns the task
• Main session title should be isolated from child agent task state
• Alternatively: provide a /title clear or /title reset command to manually reset the statusline label