Add sssf skill, installable via the skills CLI

Port the sssf skill from ~/.agents/skills/sssf into this repo so it can be
distributed and installed with the skills CLI (skills add INDigitalStudio/skills
--skill sssf).

- Copy the skill (SKILL.md, cookbooks, references, scripts, templates, and the
  visualizer app source) into sssf/.
- Gitignore build/runtime artifacts: the visualizer's node_modules/ and dist/,
  Python bytecode, and the machine-specific repos.json.
- Make the skill location-independent: install.py now stamps the skill's real
  path into the stamped justfile's skill_dir (replacing the hardcoded
  ~/.agents/skills/sssf), so 'just obs' finds the visualizer wherever the CLI
  installed the skill.
- Update cookbooks to use <skill>/scripts/... instead of the hardcoded path,
  and document the skills CLI install command.
- Update the repo README with install instructions.

sssf/apps/visualizer/vite.config.ts

@@ -0,0 +1,28 @@
+import { fileURLToPath, URL } from "node:url";
+import { defineConfig } from "vite";
+import vue from "@vitejs/plugin-vue";
+
+const API_PORT = process.env.PORT ?? "4600";
+
+export default defineConfig({
+  plugins: [vue()],
+  resolve: {
+    alias: {
+      "@": fileURLToPath(new URL("./src", import.meta.url)),
+      "@shared": fileURLToPath(new URL("./shared", import.meta.url)),
+    },
+  },
+  server: {
+    port: 4601,
+    proxy: {
+      "/api": {
+        target: `http://localhost:${API_PORT}`,
+        changeOrigin: true,
+      },
+    },
+  },
+  build: {
+    outDir: "dist",
+    emptyOutDir: true,
+  },
+});