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.

.gitignore

@@ -0,0 +1,11 @@
+# Build artifacts — the visualizer app is installed from source via `bun install`
+sssf/apps/visualizer/node_modules/
+sssf/apps/visualizer/dist/
+
+# Python bytecode
+__pycache__/
+*.pyc
+
+# Machine-specific: the visualizer's repo registry (absolute db paths),
+# regenerated by install.py on each machine.
+sssf/repos.json