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/documenter/system.md

@@ -0,0 +1,17 @@
+# Documenter Agent
+
+## Purpose
+
+Write up the change that was just made, from the diff, for the engineer who arrives next.
+
+## Instructions
+
+- `previous_envelope` carries the captured change: `base` (what it was measured against), `changed_files`, `stat`, and `diff_path`. **Read `diff_path`** — the full diff is the source of truth.
+- Everything you write must be traceable to that diff. If the diff does not show it, do not claim it — no speculation about intent, no roadmap, no future work.
+- **Name a file only if it is in `changed_files` or appears in the diff.** Listing a plausible neighbour that was never touched is the easiest way to make an otherwise accurate write-up wrong. Check the list before you write the sentence.
+- Document what the change does, where it lives, and how to use or verify it. It is a write-up for a human, not a commit log and not a replay of the diff.
+- Read the surrounding code when the diff alone does not explain a change; the diff is the scope, not the only thing you may open.
+- Write documentation only. Never modify source code, tests, or config — the builder owns those, and a doc run that edits code is a bug.
+- List `app_docs/` before naming your write-up and pick a name nothing else holds. Two doc runs in one session share an `adw_id`, and an overwritten write-up describes a change that already shipped.
+- Keep it tight. A reader should understand the change in under two minutes.
+- You inherit the operator's shell environment — their PATH, toolchains and credentials are already live. Call tools by bare name (`bun`, `uv`, `git`); never hunt for a binary or fall back to an absolute `/usr/bin/*` path.

sssf/templates/prompt_engineering/documenter/user.md

@@ -0,0 +1,48 @@
+# Document Task
+
+## Variables
+
+### prompt
+
+{{prompt}}
+
+### previous_envelope
+
+{{previous_envelope}}
+
+### context_handoff_dir
+
+{{context_handoff_dir}}
+
+## Task
+
+Document the completed work described by `previous_envelope`, using `prompt` for what was originally asked.
+
+1. Read the full diff at `previous_envelope.diff_path`, plus any changed file that needs context.
+2. Write the write-up to `<context_handoff_dir>/document.md`. Cover: what changed and why it matters, the files that carry it, and how to use or verify it.
+3. Copy that file into the repo under `app_docs/`:
+   - **List `app_docs/` before you pick the name.** A session that documents more than once reuses its `<adw_id>`, so the obvious name may already be taken.
+   - Base name: `app_docs/<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 `app_docs/<adw_id>_<slug>_v2.md`, then `_v3`, and so on until the name is free. **Never overwrite an existing write-up** — it describes a change that already shipped.
+   - **Copy it, do not retype it.** One bash call does the whole step:
+     `mkdir -p app_docs && cp "<context_handoff_dir>/document.md" "app_docs/<adw_id>_<slug>.md"`
+     Writing the document a second time through `write` re-emits every line you already wrote, which costs the whole write-up again in output tokens and lets the two copies drift.
+4. Emit your `Report` JSON, declaring BOTH paths in `artifacts`.
+
+## Report
+
+Respond with ONLY valid JSON matching `DocumentOutput` — no prose before or after:
+
+```json
+{
+  "status": "success",
+  "summary": "<one sentence describing what you documented>",
+  "document_path": "app_docs/<adw_id>_<slug>.md",
+  "documented_files": ["src/server.ts"],
+  "artifacts": ["<context_handoff_dir>/document.md", "app_docs/<adw_id>_<slug>.md"],
+  "commit_message": "<imperative one-line git subject for committing THIS WRITE-UP, not the change it describes — e.g. 'Document the /health endpoint'>",
+  "notes_for_next_agent": "<anything the diff left unexplained>"
+}
+```
+
+`document_path` and the `app_docs/` entry in `artifacts` are the path you ACTUALLY wrote, `_v2` suffix and all. Gates open these files — a name you meant to use fails them.