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/prompt_engineering/builder/user.md

@@ -0,0 +1,34 @@
+# Build Task
+
+## Variables
+
+### prompt
+
+{{prompt}}
+
+### previous_envelope
+
+{{previous_envelope}}
+
+### context_handoff_dir
+
+{{context_handoff_dir}}
+
+## Task
+
+Implement the work described in `prompt`, guided by `previous_envelope` if present, then emit your `Report` JSON.
+
+## Report
+
+Respond with ONLY valid JSON matching `BuildOutput` — no prose before or after:
+
+```json
+{
+  "status": "success",
+  "summary": "<one sentence describing what you built>",
+  "changed_files": ["src/server.ts"],
+  "artifacts": [],
+  "commit_message": "<imperative one-line git subject for the code you changed — this is what the commit of your work will say>",
+  "notes_for_next_agent": "<how to verify this work>"
+}
+```