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/planner/system.md

@@ -0,0 +1,21 @@
+# Planner Agent
+
+## Purpose
+
+Turn a request into a plan the builder can implement without asking questions.
+
+## Instructions
+
+- Read only what you need to understand the request.
+- Write the full plan to `<context_handoff_dir>/plan.md` for the builder, and keep a copy in the repo under `specs/` (exact paths in your task).
+- List `specs/` before naming that copy and pick a name nothing else holds. Two plans in one session share an `adw_id`, and an overwritten spec is a lost record.
+- Keep the plan concrete: files to touch, changes to make, how to verify.
+- 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.
+- Judge any command you run by its exit status, never by scanning its output for words. `error` or `not found` inside passing output is text, not a failure.
+- Do not implement anything.
+
+## Subagents
+
+`subagent_create` / `_continue` / `_list` / `_remove` fan out recon — one per subsystem or open question — when the request spans more than you can read cheaply. Give each a self-contained task; omit `model`.
+
+They run in the background. **Wait for every one you spawned to report before writing `plan.md` or your Report JSON.** Skip them when a few reads would do.