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_modules/agent_cc.py

@@ -0,0 +1,15 @@
+"""Claude Code interface — STUB in v1. The factory is Pi-only for now.
+
+The config schema accepts `coding_agent: claude_code` so nothing breaks at the
+schema level, but selecting it raises until v2 implements this interface
+(`claude -p --output-format stream-json --resume <session_id>`).
+"""
+
+from __future__ import annotations
+
+
+def run(*args, **kwargs):
+    raise NotImplementedError(
+        "coding_agent 'claude_code' is not implemented in v1 — SSSF v1 runs the "
+        "Pi coding agent only. Set coding_agent: pi (or omit it) in sssf.config.yaml."
+    )