docs: add CLAUDE.md with dev guide for build pipeline and test runner

Documents the dist-vs-source distinction that caused the memoriesSection
fix to not take effect, the c8 coverage runner process leak, and the
template variable maintenance contract.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,69 @@
+# Claude Code — Dev Guide for singularity-foundry
+
+## Build pipeline (MUST READ before editing extension source)
+
+Source TypeScript files under `src/resources/extensions/sf/` are **not loaded
+directly at runtime**. The loader (`src/loader.ts`) resolves extension entry
+points from `dist/resources/extensions/sf/` (compiled `.js`) and copies them
+to `~/.sf/agent/extensions/sf/` via `initResources`. Editing a `.ts` source
+file has **no effect** until you recompile:
+
+```bash
+npm run copy-resources   # tsc --project tsconfig.resources.json + file copy
+```
+
+This clears and rebuilds `dist/resources/` in one shot. Expect ~60–90 s on
+first run; subsequent runs reuse tsc's incremental cache if you keep one.
+
+The `dist-redirect.mjs` resolver (used by tests and `dev-cli.js`) only
+redirects `.js → .ts` for imports whose `parentURL` is inside `/src/`. Files
+loaded from `~/.sf/agent/extensions/sf/` (compiled JS) are **not** redirected.
+
+## Running tests
+
+**Use the lightweight `--test` runner, not `npm run test:coverage`.**
+
+The coverage runner (`c8` + `--cpu-prof` + `--heap-prof`) spawns 10–15 heavy
+worker processes per invocation. If a background run is killed or times out,
+those workers are left alive, saturating all CPUs (~700% observed).
+
+```bash
+# Run a specific test file (fast, no coverage overhead):
+node --import ./src/resources/extensions/sf/tests/resolve-ts.mjs \
+     --experimental-strip-types \
+     --test src/resources/extensions/sf/tests/<name>.test.ts
+
+# Run the full SF extension test suite:
+npm test
+```
+
+If the machine feels slow, check for stray workers:
+```bash
+ps aux | grep "heap-prof-interval" | grep -v grep
+# Kill them: ... | awk '{print $2}' | xargs kill -9
+```
+
+## Key directories
+
+| Path | Purpose |
+|------|---------|
+| `src/resources/extensions/sf/` | Extension TypeScript source (edit here) |
+| `dist/resources/extensions/sf/` | Compiled output (rebuilt by `copy-resources`) |
+| `~/.sf/agent/extensions/sf/` | Installed copy (synced from dist on startup) |
+| `src/resources/extensions/sf/prompts/` | Prompt templates (`.md`) |
+| `src/resources/extensions/sf/tests/dist-redirect.mjs` | Module resolver hook for tests |
+
+## Template variables
+
+When adding a new `{{variable}}` to a prompt template in `prompts/`, you must:
+
+1. Pass it in every `loadPrompt("template-name", { ..., newVar })` call site
+   (`auto-prompts.ts` is the main one for execute-task).
+2. Add it (with a sensible placeholder value) to any test that calls
+   `loadPrompt("template-name", {...})` — see
+   `src/resources/extensions/sf/tests/plan-slice-prompt.test.ts`.
+3. Run `npm run copy-resources` to land the change in dist.
+
+`loadPrompt` throws at runtime if any `{{var}}` in the template has no
+corresponding key in the vars object — this is intentional to catch
+template/code drift early.