Add factory:setup + upfront prerequisite checks for pnpm factory:go

[lukemelia]

Problem

On a fresh pnpm install-only checkout, pnpm factory:go could not run end-to-end. It failed with a chain of four distinct blockers, each surfacing as a separate opaque crash from deep inside a run, and each requiring monorepo-internals knowledge to fix:

1. No working Node pin — root package.json requires node >=24, but the only pin was .nvmrc (Volta doesn't read it); software-factory's volta.extends: "../../package.json" dead-ended because the root had no volta block.
2. Unbuilt boxel-cli/dist/api.js — the entrypoint imports @cardstack/boxel-cli/api, which resolves to a gitignored dist/api.js that nothing in the factory:go path builds → ERR_MODULE_NOT_FOUND at module load.
3. Host build lacking dev-mode test entries — boxel test's harness needs host/dist/tests/index.html, only emitted by a dev-mode host build.
4. Missing Playwright browser binary — boxel test launches headless Chromium; the binary was never downloaded.

None were documented, and no error pointed at the next blocker in the chain.

Fix

Attacks the root cause (factory:go did zero upfront validation) rather than committing build artifacts:

• src/preflight.ts (new) — detects all four prerequisites. Imports nothing from boxel-cli, so it can run before the dist/api.js import that used to crash first.
• src/cli/factory-entrypoint.ts — runs preflight up front and defers the boxel-cli import until it passes. Missing prerequisites produce one consolidated message pointing at pnpm factory:setup; every missing item is listed with its individual fix command. --help still works.
• scripts/factory-setup.ts + factory:setup script (new) — idempotent bootstrap that builds the missing artifacts and installs the Playwright browser, skipping anything already present. --force rebuilds everything.
• Root package.json — added a real volta pin (node: 24.17.0, matching .nvmrc/devEngines) so the existing volta.extends chain resolves.
• README — documented the one-time setup and the Node requirement.

Verification

• Preflight self-check reports the correct satisfied/missing set and formats the consolidated message.
• --help → usage, exit 0.
• Simulated missing dist/api.js → clean consolidated message + exit 1 (no ERR_MODULE_NOT_FOUND).
• factory:setup no-op run skips all three steps, exit 0.
• eslint + prettier clean; ember-tsc --noEmit clean.

Out of scope

This handles missing artifacts on a fresh checkout. A stale dist/api.js (present but built from older source) still passes the existence check yet can crash on a missing named export; factory:setup --force fixes it, but automatic staleness detection is a possible follow-up.

🤖 Generated with Claude Code

[github-actions]

Host Test Results

    1 files  ±    0      1 suites  ±0   2h 26m 1s ⏱️ + 2h 15m 25s
3 355 tests +3 203  3 340 ✅ +3 189  15 💤 +14  0 ❌ ±0 
3 373 runs  +3 221  3 358 ✅ +3 207  15 💤 +14  0 ❌ ±0 

Results for commit f582171. ± Comparison against earlier commit b57fec9.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   12m 58s ⏱️ + 1m 21s
1 934 tests +1  1 934 ✅ +1  0 💤 ±0  0 ❌ ±0 
2 013 runs  +1  2 013 ✅ +1  0 💤 ±0  0 ❌ ±0 

Results for commit f582171. ± Comparison against earlier commit b57fec9.