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/user.md

@@ -0,0 +1,45 @@
+# Plan Task
+
+## Variables
+
+### prompt
+
+{{prompt}}
+
+### previous_envelope
+
+{{previous_envelope}}
+
+### context_handoff_dir
+
+{{context_handoff_dir}}
+
+## Task
+
+Plan the work described in `prompt`.
+
+1. Write the full plan to `<context_handoff_dir>/plan.md` — this is the copy the builder reads.
+2. Copy that file into the repo under `specs/`:
+   - **List `specs/` before you pick the name.** A session that plans more than once reuses its `<adw_id>`, so the obvious name may already be taken.
+   - Base name: `specs/<adw_id>_<slug>.md`, where `<adw_id>` is the session directory name inside `context_handoff_dir` (`.../sessions/<adw_id>/context_handoff`) and `<slug>` is two to four kebab-case words naming the work.
+   - If a file with that name already exists, use `specs/<adw_id>_<slug>_v2.md`, then `_v3`, and so on until the name is free. **Never overwrite an existing spec** — the earlier plan is the record of what was asked for then.
+   - **Copy it, do not retype it.** One bash call does the whole step:
+     `mkdir -p specs && cp "<context_handoff_dir>/plan.md" "specs/<adw_id>_<slug>.md"`
+     Writing the plan a second time through `write` re-emits every line you already wrote, which costs the whole document again in output tokens and lets the two copies drift.
+3. Emit your `Report` JSON, declaring BOTH paths in `artifacts`.
+
+## Report
+
+Respond with ONLY valid JSON matching `PlanOutput` — no prose before or after:
+
+```json
+{
+  "status": "success",
+  "summary": "<one sentence describing the plan>",
+  "artifacts": ["<context_handoff_dir>/plan.md", "specs/<adw_id>_<slug>.md"],
+  "commit_message": "<imperative one-line git subject for committing THIS PLAN DOCUMENT, not the work it describes — e.g. 'Add spec for the /health endpoint'>",
+  "notes_for_next_agent": "<what the builder must know>"
+}
+```
+
+Both `artifacts` entries are the paths you ACTUALLY wrote, `_v2` suffix and all. Gates open these files — a name you meant to use fails them.