feat(sandcastle): scaffold .sandcastle/ + README + env example

.sandcastle/.env.example

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+# Anthropic Claude API
+ANTHROPIC_API_KEY=
+
+# OpenAI / Codex
+OPENAI_API_KEY=
+
+# GitHub (for PR creation by the orchestrator)
+GITHUB_TOKEN=
+
+# Sandcastle's own configuration — sandbox provider (docker | podman | vercel | custom)
+SANDCASTLE_PROVIDER=docker

.sandcastle/.gitignore

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+.env
+*.log
+.cache/

.sandcastle/README.md

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+# .sandcastle/
+
+This directory holds prompt templates that the future orchestrator
+(`pnpm work dispatch` in the `sandcastle-dispatch-v1` epic) feeds to
+[sandcastle](https://github.com/mattpocock/sandcastle) when dispatching
+agents.
+
+## Prompt templates
+
+| File                     | Role                                      | Variables                           |
+| ------------------------ | ----------------------------------------- | ----------------------------------- |
+| `prd-eliciter.prompt.md` | Interview a human to produce a PRD draft  | `{{INITIAL_BRIEF}}`                 |
+| `adr-eliciter.prompt.md` | Interview a human to produce an ADR draft | `{{INITIAL_PROPOSAL}}`              |
+| `decomposer.prompt.md`   | Turn a PRD into epic + story files        | `{{PRD_FILE_CONTENT}}`              |
+| `implementer.prompt.md`  | Execute a single task                     | `{{TASK_FILE_CONTENT}}`             |
+| `reviewer.prompt.md`     | Review the implementer's diff             | `{{TASK_FILE_CONTENT}}`, `{{DIFF}}` |
+
+## Convention: every prompt enforces "generators first"
+
+Each prompt template starts with the same non-negotiable rule: **the agent
+must prefer `pnpm turbo gen <kind>` over hand-rolled scaffolding.** This
+applies to feature packages, events, jobs, realtime channels, optional
+core packages, and atomic-design components. Hand-rolled code is only
+acceptable when the generator's output doesn't cover the case — and even
+then, the agent runs the generator first and modifies its output rather
+than starting from scratch.
+
+## Environment
+
+Configure runtime tokens via `.env` (gitignored). Copy `.env.example`
+and fill values for the providers you use.
+
+## Manual usage
+
+Until the orchestrator ships, these templates are usable manually: copy
+the relevant `.prompt.md` content into a Claude / Codex / other agent
+session, fill the `{{VARIABLE}}` placeholders by hand, and run.