scripts: forge-driven fix-loop launcher (git→gitea→agentic→tmux)

First live test of the sx-forge technology driving a real work session:
- sx-fix-up.sh <forge-agent> <briefing.md>: reads the agent's briefing FROM
  the rose-ash/sx-review forge (agentic-sx branch), materialises a git
  worktree + branch (loops/sx-<slug>), and spins up a tmux+claude session
  briefed from the forge. Commits are LOCAL by default (no push).
- sx-fix-down.sh [--clean]: stop the sx-fix session; --clean removes worktrees.
- plans/agent-briefings/sx-gate-loop.md: W14 (test gate) briefing — the safe
  first payload (test-only, cannot regress the 5762p/274f baseline), scoped
  commit-no-push with hard guardrails.

Verified live: launcher read the W14 briefing from the forge, created worktree
/root/rose-ash-loops/sx-ws-w14 on loops/sx-ws-w14, booted claude, and the agent
picked up the briefing. Watch: tmux a -t sx-fix. Note: MCP servers need /mcp
auth in a fresh worktree (agent works via Bash meanwhile).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/sx-fix-down.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+# sx-fix-down.sh — stop the sx-fix tmux session (and optionally remove worktrees).
+# Usage: ./scripts/sx-fix-down.sh [--clean]
+#   --clean  also `git worktree remove` every /root/rose-ash-loops/sx-* worktree
+# Commits on loops/sx-* branches are preserved either way (branches are not deleted).
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"; cd "$ROOT"
+SESSION="sx-fix"
+
+if tmux has-session -t "$SESSION" 2>/dev/null; then
+  tmux kill-session -t "$SESSION"
+  echo "killed tmux session '$SESSION'."
+else
+  echo "no tmux session '$SESSION'."
+fi
+
+if [ "${1:-}" = "--clean" ]; then
+  for wt in /root/rose-ash-loops/sx-*; do
+    [ -d "$wt" ] || continue
+    git worktree remove --force "$wt" 2>/dev/null && echo "removed worktree $wt" || echo "skip $wt"
+  done
+  echo "branches loops/sx-* are kept (commits preserved)."
+fi