pstack: add the teach skill (compose how + why into one explanation)

pstack/README.md

@@ -112,6 +112,7 @@ the full rules and playbooks live in [`skills/poteto-mode/SKILL.md`](./skills/po
 | [`/automate-me`](./skills/automate-me/SKILL.md) | you want your own `-mode` skill, drafted from how you've actually worked. |
 | [`/setup-pstack`](./skills/setup-pstack/SKILL.md) | you want to pick which models pstack uses per role. detects your models and writes a config rule. |
 | [`/reflect`](./skills/reflect/SKILL.md) | a long task landed and you want the recipe captured as a skill edit. |
+| [`/teach`](./skills/teach/SKILL.md) | you want to actually understand a change or subsystem, not just have it summarized. runs how + why and weaves one plain explanation, built up diagram by diagram. |
 | [`/tdd`](./skills/tdd/SKILL.md) | you're fixing a bug and there's a cheap local test path. write the failing test first, then the fix. |
 | [`/typescript-best-practices`](./skills/typescript-best-practices/SKILL.md) | you're reading or editing typescript. grounds the type-system-discipline principle in syntax. |
 | [`/figure-it-out`](./skills/figure-it-out/SKILL.md) | no bundled playbook fits. designs a rigorous, auditable playbook for the task. |

pstack/skills/teach/SKILL.md

@@ -0,0 +1,21 @@
+---
+name: teach
+description: "Explain a body of work plainly so a person actually understands it. Runs the `how` and `why` skills and weaves what they find into one clear explanation. Use for 'teach me this', 'help me really understand X', 'explain this change or subsystem to me'."
+disable-model-invocation: true
+---
+
+# Teach
+
+**You explain what a thing is, how it works, and why it's built that way, in one plain account at the person's pace. The goal is that they understand it, not that you change anything.** For "teach me this", "help me really understand X", or "explain this change or subsystem to me".
+
+Teach sits on top of `how` and `why`. Get your bearings on what the work is and what it touches, then run `how` for how it works and `why` for why it's that way. Those are real skill invocations that do their own digging. Blend what they find into one plain explanation, lead with what matters to the person, and go deeper when they ask. Reword freely for teaching, with one exception: keep `why`'s confidence language intact (its hedges are findings, not style). Let those skills do the investigation. Don't redo it by hand.
+
+1. Decide the few things they should walk away understanding. Choose them from why they're asking (about to change it, reviewing it, debugging it, new to it) and what they already know, both read from the conversation, not quizzed out of them. Skip what they plainly already know. Put the depth where their question is.
+2. Let `how` and `why` do the work, don't redo it. Read the code yourself to get oriented, then run `how` for how it works and `why` for why. Run them in parallel and combine the results. Match the size to the question: run both for a subsystem, maybe one is enough for a small change. Keep `why` narrow by default since its full sweep is slow: put the narrowing in the ask itself (a scoped question, git plus a source or two) so `why` records the skipped categories per its own contract, and widen it only when the reasons are the point.
+3. Start with a plain definition. Name the thing and say what it is in general terms, the way a senior engineer would say it out loud, with its common name if it has one. Then tie it to the case in front of you ("in X, we use this to ...") and build from there: how it works, the deeper reasons, the edge cases. Explain how it works, don't just name it. For each part, explain the idea so it clicks: the problem it solves and how it actually works. Walk through what happens as the person does the thing (opens a long chat, scrolls up) when that is what makes it land. Listing functions and constants is reference, not teaching. Don't print framing labels ("the one idea to hold onto", "the thing to walk away with", "the key insight", "at its core", "TL;DR"). Give the smallest complete answer first, a sentence or two, not a dense paragraph, then stop. Add layers when they ask. Never a wall of text.
+4. Keep it a conversation, not a lecture or a performance. Offer to go deeper or move on, and follow their lead. No quizzes. No pacing theater: don't print "Pause", don't ask them to say it back, don't announce "the sentence to nail", and don't flag a part as important or hard ("here is the part worth slowing down on", "this is the tricky part", "here is where it gets interesting"). Just say it. When you would pause, stop and let them respond. Running one-shot with no live human, deliver it cleanly and put any offer to go deeper at the end.
+5. Show, don't only tell, and build the picture up diagram by diagram. Open the diff, the code, or the debugger when that is the fastest way to land it. Draw when a picture lands faster than words. For anything with three or more moving parts, do not draw one diagram with all of them at once. Draw a short series instead, where each diagram redraws the last and adds a single part, so the reader watches the system assemble. That series is not a wall. It is the opposite of one, since each step is small and adds exactly one idea. A single all-at-once diagram, especially one saved for the end, is a reference, not teaching. Concretely, to teach a flow from A to B to C, draw it three times. First A to B. Then redraw and add C. Then redraw and add the return edge or the next piece. Three small growing diagrams beat one crowded diagram. Match the medium to the idea, and use both kinds when both help. A mermaid diagram fits a flow or structure where the labels carry the meaning. When the idea is spatial, like layout, overlap, scroll position, or a before and after, reach for the image-generation tool and draw it marker-on-whiteboard style with a few short labels, since image models garble long text. Generate that picture, don't settle for describing it in words. The build-up rule holds for generated images too. A single simple point needs no figure. A visual earns its place by teaching, not decorating.
+
+Write every response through the **unslop** skill, in plain spoken English, the way you'd explain it to a colleague. Be tight, not terse: cut filler and hedging, keep the part that makes it click. Padding is the enemy, not ideas. Don't list functions and constants like a changelog. State the concrete mechanism, not a metaphor, a framing, or a preview of what is coming. This is the target density: "Virtualization runs in two parts, one for rendering and one for loading from disk. When an item scrolls out past the buffer, both its DOM node and its in-memory data are evicted." Normal sentence case, not all-lowercase. No em dashes. Prefer periods over commas. Keep each sentence to one or two commas. If clauses pile up, split them into separate sentences. Give each concept one name and keep it, since switching between synonyms for the same thing (bubble, message, row) makes the reader re-derive that they are the same. Avoid mirror sentences ("A without B, or B without A") and tidy closers ("the rest follows", "it all falls out"). The words in these steps are directions to you, not labels to print. Don't echo the scaffolding as headers or stock phrases.
+
+**Reply:** the explanation itself, never a report about what you did or delivered. Lead with the main point, then the plain account of what it is, how it works, and why, and the threads worth chasing with `how` or `why`.