fix: shell injection via branch names + feature-branch sampling bias

Codex code review found two issues:

P1: eval $(gstack-slug) in gstack-repo-mode executes branch names as
shell. Branch names like foo$(touch${IFS}pwned) are valid git refs and
would execute arbitrary commands. Fix: compute SLUG directly with sed
instead of eval'ing gstack-slug output.

P2: git shortlog HEAD only sees current branch history. On feature
branches that haven't merged main recently, other contributors disappear
from the sample. Fix: use git shortlog on the default branch
(origin/main) instead of HEAD.

Also improved blame lookup in collaborative triage to check both the
test file and the production code it covers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/gstack-repo-mode

@@ -12,7 +12,8 @@
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-eval $("$SCRIPT_DIR/gstack-slug" 2>/dev/null) || { echo "REPO_MODE=unknown"; exit 0; }
+# Compute SLUG directly (avoid eval of gstack-slug — branch names can contain shell metacharacters)
+SLUG=$(git remote get-url origin 2>/dev/null | sed 's|.*[:/]\([^/]*/[^/]*\)\.git$|\1|;s|.*[:/]\([^/]*/[^/]*\)$|\1|' | tr '/' '-')
 [ -z "${SLUG:-}" ] && { echo "REPO_MODE=unknown"; exit 0; }
 
 # Validate: only allow known values (prevent shell injection via source <(...))
@@ -39,7 +40,9 @@ if [ -f "$CACHE_FILE" ]; then
 fi
 
 # Compute from git history (90-day window)
-SHORTLOG=$(git shortlog -sn --since="90 days ago" --no-merges HEAD 2>/dev/null | head -20)
+# Use default branch (not HEAD) to avoid feature-branch sampling bias
+DEFAULT_BRANCH=$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's|refs/remotes/||' || echo "origin/main")
+SHORTLOG=$(git shortlog -sn --since="90 days ago" --no-merges "$DEFAULT_BRANCH" 2>/dev/null | head -20)
 if [ -z "$SHORTLOG" ]; then
   echo "REPO_MODE=unknown"
   exit 0