docs: fix local dev setup (dasel v2, supabase via pnpm, JWT secret)

[devin-ai-integration]

Summary

The current local dev setup has several issues that need to be addressed.

Issues

1. dasel v3 incompatibility

scripts/setup-devtools.sh installs the latest dasel release, which is now v3. However, dasel v3 does not support the flags used in Taskfile.yaml (e.g. dasel put -f ... -t int -v 17 ...), which are v2 syntax.

Fix: Update the docs/scripts to download the latest dasel v2 binary instead of instructing users to install dasel v3.

2. Run supabase via pnpm

Supabase CLI is currently installed as a standalone binary in setup-devtools.sh. It should be run via pnpm instead, and supabase should be added to onlyBuiltDependencies.

3. Supabase container fails to start

When running supabase start, the container does not come up. The rest-admin health check fails with:

postgrest: FatalError {fatalErrorMessage = "user error (The JWT secret must be at least 32 characters long.)"}