Add a command-by-command Slack walkthrough to the docs

docs/cookbook/07-slack.md

@@ -7,6 +7,9 @@ is enough. A workspace member types `/grapharc metrics t.jsonl r1` (or
 mentions the bot in a channel); the bot runs the command on the host and posts
 the output back in the thread.
 
+Setup lives here; what a real session looks like, exchange by exchange, is
+[08-slack-walkthrough.md](08-slack-walkthrough.md).
+
 Nothing in this page is byte-compared by the test suite — Slack is on the
 other end of every interesting command. What *is* tested, in
 `tests/test_slack_gateway.py`, is everything short of Slack itself: the gate

docs/cookbook/08-slack-walkthrough.md

@@ -0,0 +1,89 @@
+# A Slack session, command by command
+
+[07-slack.md](07-slack.md) covers setting the bot up; this page is what using
+it actually looks like. Every exchange below is from a real session — the
+first hour of driving `grapharc` from Slack — including the refusals and the
+mistakes, because those teach the model of the tool faster than the successes.
+
+One rule carries the whole page: **one message is one command.** The bot
+treats the entire message as a single command line; pasting three commands in
+one message glues them into one argv and the CLI rejects it (exit 2). Send
+commands one at a time, and wait for a long one to answer before reading its
+trace.
+
+## First contact
+
+```
+/grapharc models
+```
+
+Instant, offline, spend-free — the right first ping. You get the backend
+table and which specs need a key. `/grapharc` with nothing after it returns
+the allowed-command list.
+
+If the slash command says *not a valid command*, the app needs a reinstall
+after the command was registered; mentions (`@grapharc models`) work as soon
+as the bot is invited to the channel.
+
+## The run → read loop
+
+The shape of everything else: run something with a **named trace and run id**,
+then read that trace back.
+
+```
+@grapharc demo stage2 --trace demo.jsonl --run-id first-demo
+@grapharc trace demo.jsonl
+@grapharc metrics demo.jsonl first-demo
+@grapharc viz demo.jsonl first-demo
+```
+
+Name both, always. An unnamed run mints a random id you then have to fish out
+of the trace, and two runs pointed at the same `--trace` file interleave their
+events in one JSONL — legal, but confusing to read. Fresh file, explicit id,
+every run.
+
+`trace` is the audit log — every event, verbatim. `metrics` is the summary:
+tokens, durations, per-node counts, termination reason. `viz` prints the
+executed path as Mermaid, and the reply ends with a *render this diagram*
+link that draws it in your browser (the diagram travels compressed inside the
+URL fragment, which a browser sends to no server).
+
+## Planning with a real model
+
+The bot's default is spend-free: `--model` is refused until the operator
+restarts the bot with `GRAPHARC_SLACK_ALLOW_MODEL=1` — a decision made in the
+shell that starts the bot, deliberately not from Slack. Once it's on:
+
+```
+@grapharc plan "investigate improvements for the readme file" --model claude-cli/claude-sonnet-5 --trace plan.jsonl --run-id readme-1
+```
+
+A scripted `plan` answers in under a second; a real one takes tens of
+seconds, and the mention reply arrives only when it finishes. Reading the
+trace afterwards is where the governance becomes visible: rounds where the
+model's reply had no parseable proposal, a structurally valid proposal
+rejected by the edge policy (`policy/edge_denied`), and finally an admitted
+round that executed. Every model failure contained, every decision recorded.
+
+## What a refusal looks like, and why
+
+Two refusals from the same session, both correct:
+
+- `@grapharc agent "do something"` → *not a command this bot runs*. `agent`
+  executes tools on the host on behalf of anyone in the workspace, so it is
+  excluded at the gate, not hidden.
+- `plan "make a new file for the docs" --model …` → **ran; the answer was
+  negative**. The planner may only propose node kinds from its registry —
+  the incident-response demo set — and none of them can create a file. The
+  model found no admissible work and said so, which is exit 1 doing its job:
+  a weaker loop would have executed three irrelevant nodes and called it
+  success. File-creation belongs to `grapharc agent`, on the host.
+
+## The paths rule
+
+Every path in a command must resolve inside the bot's working directory —
+`trace ../somewhere-else.jsonl` is refused before a process spawns. The
+corollary: tools that default their output *outside* the workdir (a bare
+`plan` writes its trace to a temp directory) leave you with a file Slack
+cannot read back. Passing `--trace <name>.jsonl` keeps the whole
+run-then-read loop inside the bot's world, which is the point of it.