feat: add dev-install script for macOS

[graydawnc]

Summary

Add a one-shot developer workflow for building and installing Spool from source on Apple Silicon macs, so contributors can quickly smoke-test a production build without copy-pasting commands.

• scripts/dev-install-mac.sh — builds via pnpm -F @spool/app build:mac, quits any running Spool, replaces /Applications/Spool.app, strips the com.apple.quarantine attribute (unsigned local builds would otherwise be blocked by Gatekeeper), then launches the app.
• pnpm dev:install:mac shortcut added to the root package.json.
• CONTRIBUTING.md gets a short section explaining when to use it.

Design notes

• Platform-specific naming on purpose. The script is explicitly dev-install-mac.sh rather than a cross-platform script with uname branching. All current contributors are on macOS; a sibling dev-install-linux.sh can be added when we onboard a Linux developer, without touching the mac path.
• Apple Silicon only. Matches the existing build:mac target (--arm64). Fails fast with a clear error on Intel or non-Darwin.
• Not wired into CI — this is a local DX tool, not a release artifact. The production installer for end users still lives at packages/landing/public/install.sh and is unaffected.

Test plan

• Run pnpm dev:install:mac on a clean checkout on an M-series mac
• Verify /Applications/Spool.app is replaced and launches without a Gatekeeper prompt
• Re-run with Spool already open — it should quit and relaunch cleanly

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