[FEATURE] Non-interactive attach to a background agent: `claude agents attach <id|name>`

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When running several background agents (dispatched via claude agents or as background jobs), jumping into a specific one currently requires opening the interactive agent view (claude agents), scanning the list, and selecting the session by hand. There is no way to attach to a known background agent directly from the shell.

This makes it awkward to:

• script "jump back into agent X" workflows (shell aliases, tmux/wezterm keybindings, fuzzy-finder pickers built on claude agents --json)
• attach quickly when you already know exactly which agent you want (by name, ID, or worktree path)

The closest thing today is claude --resume <sessionId>, but per the sessions docs that reopens the transcript rather than attaching to the live managed session the way the agent view does, and resuming a session that's actively running in another process interleaves two writers into one transcript.

Proposed Solution

A non-interactive attach entry point, e.g.:

claude agents attach <id|name>
# or
claude attach <id|name>

that does exactly what selecting the session in the claude agents view does (attach to the live session, ← to detach), but skips the picker. Matching by the short agent id, name, or sessionId from claude agents --json would all be useful; a --cwd <path> matcher would also fit nicely with the existing claude agents --cwd filter.

Alternative Solutions

• claude agents --json | jq ... to find the sessionId, then claude -r <sessionId> — works, but it's transcript resume rather than a true attach to the live managed session, with the dual-writer interleaving caveat.
• tmux-style tools solve this with tmux attach -t <name>; same ergonomics would be great here.

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

1. I have 4 background agents working on separate worktrees of a monorepo.
2. One of them reports needs input: (visible in the job list / notifications).
3. Today: run claude agents, find it in the list, select it.
4. With this feature: claude agents attach dataset-filtering (or a keybinding that runs claude agents attach $(... | fzf)) and I'm in the live session immediately.