[BUG] Orphaned plugin marketplace git clone busy-loops at ~80% CPU for days after the session exits (Linux)

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

A background `git clone` of the official plugin marketplace was left orphaned (its parent became `systemd --user`) after the Claude Code session that started it exited. It busy-looped at ~80% CPU for 3 days, keeping one CPU core at 100°C and the laptop fan near max the whole time. SIGTERM was ignored; it needed `kill -9`.

- Started Sep 15 14:05, still running on Sep 18.
- Its `git index-pack --stdin` child was idle in `pipe_read` while the parent `git` spun in state R.
- The clone target folder `anthropics-claude-plugins-official` doesn't exist. The marketplace is actually installed at `~/.claude/plugins/marketplaces/claude-plugins-official` (per `known_marketplaces.json`) and had updated normally, so this clone was a leftover.

### What Should Happen?

Marketplace git operations should have a timeout, and background git processes should be killed when the session that started them exits instead of running forever.

### Error Messages/Logs

```shell
PID %CPU ELAPSED STARTED STAT
3972130 78.8 3-01:02:49 Tue Sep 15 14:05:30 2026 Rl (parent PID: systemd --user)

/usr/bin/git -c core.fsmonitor= -c core.hooksPath=/dev/null -c core.askPass= -c submodule.recurse=false -c log.showSignature=false -c gc.auto=0 -c maintenance.auto=false -c core.sshCommand=ssh -o BatchMode=yes -o StrictHostKeyChecking=yes -c protocol.ext.allow=never -c protocol.fd.allow=never -c protocol.git.allow=never -c protocol.ftp.allow=never -c protocol.ftps.allow=never clone --depth 1 --recurse-submodules --shallow-submodules -- git@github.com:anthropics/claude-plugins-official.git ~/.claude/plugins/marketplaces/anthropics-claude-plugins-official

child: /usr/lib/git-core/git --shallow-file .../anthropics-claude-plugins-official/.git/shallow.lock index-pack --stdin --fix-thin --keep=fetch-pack (State: S, wchan: pipe_read, 0% CPU)
```

### Steps to Reproduce

I couldn't reproduce it on purpose. What I observed:
1. Use Claude Code normally on Linux with the official plugin marketplace installed (several sessions open over a few days).
2. At some point a background marketplace `git clone` (over SSH) starts, and the session that launched it exits.
3. The orphaned `git clone` keeps running at ~80% CPU indefinitely (3+ days in my case) until killed with `kill -9`.
4.
Check with: ps -eo pid,etime,pcpu,args | grep "[g]it .*clone.*claude-plugins-official"

### Claude Model

None

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.276 (Claude Code)

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

Other

### Additional Information

Ubuntu 24.04, kernel 6.8, Intel Core Ultra 5 135H (Dell Precision 3591). I noticed it only because the fan never stopped for days; CPU temp dropped from 101°C to 66°C within 2 minutes of killing the process.

Start by tracing the background git clone shown in the report and compare its target with ~/.claude/plugins/marketplaces/claude-plugins-official and known_marketplaces.json. Use the provided ps command to inspect orphaned processes on Ubuntu/Debian Linux. Done means marketplace git operations time out and background processes stop when their launching session exits.

Written by the indexing model from the issue text.