fix(ui): keep repository and branch context visible after the first message #2171

Problem

The Composer workspace row is rendered only while !activeSession. After the first message creates the Session, repository, branch, working directory, and worktree identity disappear from the chat surface.

Users can no longer tell which filesystem and Git context the agent is modifying.

Expected

• New-task mode keeps editable project/branch controls.
• Existing Sessions show a persistent read-only workspace context.
• The context identifies project/repository, branch, and worktree status.
• Switching Sessions updates the context from the Session-owned cwd.
• Unavailable paths and detached HEAD are explicit.

Acceptance

• Context remains visible before and after first send.
• Existing Session context cannot silently switch its branch.
• Different worktrees render distinct context.
• Narrow layouts preserve accessible full context.