Scope prompt input history per session name (--name)

Problem

When running multiple Claude Code sessions from the same directory (e.g., using claude --name "task A" and claude --name "task B"), the up-arrow prompt history is shared between them. This creates noise — prompts from unrelated sessions interleave when navigating history.

Current behavior

Input history is stored per project directory. All sessions in the same directory share one history file regardless of their --name.

Expected behavior

When a session is started with --name, its input history should be scoped to that name. Up-arrow in "task A" should only show prompts from previous "task A" sessions, not from "task B".

Unnamed sessions could continue using the shared per-directory history as a fallback.

Use case

Power users running parallel named sessions for different work streams in the same repo (e.g., one for a feature branch, one for code review, one for debugging). The --name flag already scopes conversation resume — extending it to input history would complete the isolation.