skill-creator: run_eval.py's nested claude -p subprocess inherits parent stdin, causing an intermittent stall

## Problem

`run_single_query()` in `skills/skill-creator/scripts/run_eval.py` spawns the nested eval `claude -p` via `subprocess.Popen(cmd, stdout=..., stderr=..., cwd=..., env=...)` without redirecting `stdin`. The child inherits the parent's stdin file descriptor.

When `run_eval.py`/`run_loop.py` itself runs nested inside a `claude` session (a supported, intended use — the file explicitly strips `CLAUDECODE` from `env` to allow this), the child process can block briefly waiting on/polling the inherited stdin before it gives up and proceeds, adding an observed ~3s stall per query. At `--num-workers` > 1 this multiplies across concurrent subprocesses and inflates wall-clock eval time without any corresponding signal.

## Repro

Run any eval from inside a `claude -p` or Claude Code session:

python -m scripts.run_loop --eval-set eval.json --skill-path --model

Observe each `run_single_query` call taking ~3s longer than the same call run from a plain (non-nested) shell.

## Fix

Single-line change — explicitly discard the child's stdin instead of inheriting it:

```diff
--- a/skills/skill-creator/scripts/run_eval.py
+++ b/skills/skill-creator/scripts/run_eval.py
@@ -84,6 +84,7 @@ def run_single_query(

process = subprocess.Popen(
cmd,
+ stdin=subprocess.DEVNULL, # nested claude -p must not wait on/inherit our stdin
stdout=subprocess.PIPE,
stderr=subprocess.DEVNULL,
cwd=project_root,
```

## Note for maintainers

Neither open PR #1323 nor #1298 (both reworking this same function for the 0%-recall bug) touches the Popen call — this fix is orthogonal and should be folded into whichever lands, or applied standalone if merged first.

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*Disclosure: this issue was researched and written by Claude (Claude Code)*