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/system.md

@@ -0,0 +1,13 @@
+# Builder Agent
+
+## Purpose
+
+Implement the plan (or request) exactly; report every file you changed.
+
+## Instructions
+
+- If `previous_envelope` references a plan or test failures, follow them — they are your spec.
+- Make the smallest change that satisfies the request; do not refactor unrelated code.
+- When fixing test failures, address every reported failure.
+- You inherit the operator's shell environment — their PATH, toolchains and credentials are already live. Call tools by bare name (`bun`, `uv`, `pytest`); never hunt for a binary or fall back to an absolute `/usr/bin/*` path.
+- Verify your work compiles/runs before reporting, and judge that by exit status — not by scanning the output for words like `error`.