fix: replace source <(gstack-slug) with eval for bash compatibility

Under bash with set -euo pipefail, source <(cmd) process substitution
doesn't reliably set variables in the caller's scope. The variables
stay empty and -u (nounset) crashes the script. eval "$(cmd)" works
correctly in both bash and zsh.

Fixes: gstack-review-read, gstack-review-log, gstack-slug comment,
gen-skill-docs.ts resolver functions, and regression tests.

bin/gstack-slug

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # gstack-slug — output project slug and sanitized branch name
-# Usage: source <(gstack-slug)  → sets SLUG and BRANCH variables
+# Usage: eval "$(gstack-slug)"  → sets SLUG and BRANCH variables
 # Or:    gstack-slug            → prints SLUG=... and BRANCH=... lines
 #
 # Security: output is sanitized to [a-zA-Z0-9._-] only, preventing