Add ocap-discipline skill

ocap-discipline/SKILL.md

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+---
+name: ocap-discipline
+description: Use when writing or reviewing code whose functions touch I/O, clocks, random number generators, process.env, fetch, or filesystem; when someone says "ambient authority", "wash your mouth out", or "that function shouldn't reach for that"; when debating dependency injection or default-arg seams; when asked what OCap discipline is. Source: the Agoric wiki's OCap Discipline page.
+---
+
+# OCap Discipline
+
+[Agoric's OCap Discipline wiki page](https://github.com/Agoric/agoric-sdk/wiki/OCap-Discipline),
+condensed to working rules.
+
+## the background, in three lines
+
+Object-capability security = memory safety + encapsulation ("no
+reference to an object except: create it, be given it at creation, or
+receive it in a message") combined with **primitive effects only via
+references** — all world-touching primitives are embodied by objects,
+and *anything globally accessible is immutable data*. No `open(filename)`
+in the global namespace: converting a string to file access is like
+casting an integer to a pointer. JavaScript doesn't enforce this; the
+discipline is a convention (layerable via SES).
+
+## the working rules (from the wiki)
+
+1. **Exported functions depend only on their arguments.** I/O, clocks,
+   RNGs, `process.env`, network — injected as parameters, never
+   imported inside the function body.
+   - transitional seam: a *default argument* the caller may override
+   - modules must **not** export powerful objects (nothing closing
+     over `fs` or `process.env`)
+   - avoid static mutable state
+2. **Program scripts are the exception**: they start
+   `#!/usr/bin/env node` (and +x), are free to import all the
+   authority they need, then *delegate* it downward.
+3. **Test scripts** are named `test-*.js`; tests export nothing.
+
+## how this shows up in review
+
+- `crypto.getRandomValues()` called inside an exported function →
+  ambient RNG authority: untestable, unreplayable, uncomposable.
+  Fix: the function takes the random bytes (or an `Entropy` object)
+  as a parameter; the *caller* at the edge is who holds the ambient
+  right.
+- `new Date()` inside logic → ambient clock. Same treatment:
+  parameter, default to a live-clock implementation.
+- `fetch` / `fs` / `env` referenced directly inside a helper's body →
+  helpers become untestable without monkey-patching; inject a
+  narrower interface (the caller passes the one URL's worth of fetch,
+  not the whole world).
+- A module's default export closing over bindings/config → a
+  powerful object export. Split: pure logic (argument-only) + a thin
+  program-script-ish edge that wires authority in.
+
+## canonical fixes
+
+```ts
+// ambient: untestable
+export function mintKey(): string {
+  return b32(crypto.getRandomValues(new Uint8Array(16)));
+}
+
+// disciplined: entropy in, key out (caller holds the RNG authority)
+export const mintKey = (entropy: Entropy): string => b32(entropy(16));
+```
+
+## references
+
+- [OCap Discipline — Agoric wiki](https://github.com/Agoric/agoric-sdk/wiki/OCap-Discipline)
+  (Miller, *Robust Composition* PhD thesis, 2006, is the deep source)
+- [ambient authority — Wikipedia](https://en.wikipedia.org/wiki/Ambient_authority)
+- in this workspace: `capsite/docs/webkeys.md` (URL-borne bearers),
+  Agoric's eslit-based enforcement notes for contracts