fix: migrate all remaining eval callers to source, fix stale CHANGELOG claim

5 templates and 2 bin scripts still used eval $(gstack-slug). All now use
source <(gstack-slug). Updated gstack-slug comment to match. Fixed v0.8.3
CHANGELOG entry that falsely claimed eval was fully eliminated — it was
the output sanitization that made it safe, not a calling convention change.

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

.agents/skills/gstack-benchmark/SKILL.md

@@ -290,7 +290,7 @@ When the user types `/benchmark`, run this skill.
 ### Phase 1: Setup
 
 ```bash
-eval $(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null || echo "SLUG=unknown")
+source <(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null || echo "SLUG=unknown")
 mkdir -p .gstack/benchmark-reports
 mkdir -p .gstack/benchmark-reports/baselines
 ```

.agents/skills/gstack-canary/SKILL.md

@@ -308,7 +308,7 @@ When the user types `/canary`, run this skill.
 ### Phase 1: Setup
 
 ```bash
-eval $(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null || echo "SLUG=unknown")
+source <(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null || echo "SLUG=unknown")
 mkdir -p .gstack/canary-reports
 mkdir -p .gstack/canary-reports/baselines
 mkdir -p .gstack/canary-reports/screenshots
@@ -458,7 +458,7 @@ Save report to `.gstack/canary-reports/{date}-canary.md` and `.gstack/canary-rep
 Log the result for the review dashboard:
 
 ```bash
-eval $(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null)
+source <(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null)
 mkdir -p ~/.gstack/projects/$SLUG
 ```
 

.agents/skills/gstack-land-and-deploy/SKILL.md

@@ -840,7 +840,7 @@ Save report to `.gstack/deploy-reports/{date}-pr{number}-deploy.md`.
 Log to the review dashboard:
 
 ```bash
-eval $(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null)
+source <(~/.codex/skills/gstack/bin/gstack-slug 2>/dev/null)
 mkdir -p ~/.gstack/projects/$SLUG
 ```