Fail fast on missing env vars in fetch_reviews.sh

Without any guards the script trusted `PR_NUMBER` and the four
`NUMBER_OF_*` variables to be set.  Missing ones expanded to empty
strings, so the script silently constructed paths like
`plans//fetched/...` and forwarded empty `-F threads=`, `-F comments=`
arguments.  `gh` would then reject the call with a generic GraphQL
error instead of pointing at the missing variable, while the empty
directory had already been created.

Add `set -euo pipefail` and a `: "${VAR:?…}"` guard for each of the
five required variables, so the script aborts at the missing variable
with a clear "VAR is required" message before touching the filesystem
or talking to GitHub.

`LAST_CURSOR` is intentionally not guarded — it is the optional
incremental-fetch cursor and is referenced only through
`${LAST_CURSOR:+-F after="$LAST_CURSOR"}`, which is safe under `set -u`.

Addresses:

 -  #765 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: 2chanhaeng

.agents/skills/get-reviews/fetch_reviews.sh

@@ -1,4 +1,12 @@
 #!/usr/bin/env bash
+set -euo pipefail
+
+: "${PR_NUMBER:?PR_NUMBER is required}"
+: "${NUMBER_OF_PR_COMMENTS:?NUMBER_OF_PR_COMMENTS is required}"
+: "${NUMBER_OF_REVIEWS:?NUMBER_OF_REVIEWS is required}"
+: "${NUMBER_OF_THREADS:?NUMBER_OF_THREADS is required}"
+: "${NUMBER_OF_COMMENTS_PER_THREAD:?NUMBER_OF_COMMENTS_PER_THREAD is required}"
+
 PR_PATH="plans/$PR_NUMBER"
 FETCHED_PATH="$PR_PATH/fetched"
 mkdir -p "$FETCHED_PATH"