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/templates/adws/adw_validate.py

@@ -0,0 +1,36 @@
+#!/usr/bin/env -S uv run
+# /// script
+# dependencies = ["pydantic", "python-dotenv", "pyyaml", "rich"]
+# ///
+"""ADW Validate — check the config without running anything.
+
+Loads the roster and validates every agent: the name resolves, the coding
+agent is implemented, the prompt files exist, and the model resolves in the
+harness's catalog. Exits 0 on success, 1 with a list of problems otherwise.
+
+Usage:
+    uv run adws/adw_validate.py [--config adws/adw_sssf_config/sssf.config.yaml]
+"""
+
+import argparse
+import sys
+
+from adw_modules import agents
+
+
+def main(config: str = "adws/adw_sssf_config/sssf.config.yaml") -> int:
+    cfg = agents.load_config(config)
+    names = [a.name for a in cfg.agents]
+    if not names:
+        print(f"config {config} declares no agents")
+        return 1
+    agents.validate(cfg, names)
+    print(f"config OK; agents: {', '.join(names)}")
+    return 0
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--config", default="adws/adw_sssf_config/sssf.config.yaml")
+    args = parser.parse_args()
+    sys.exit(main(args.config))