[codex] add runtime config and database foundation

[dbtlr]

Summary

Adds the runtime config and database foundation phase: Effect-backed config loading, Drizzle/Postgres wiring, workspace registration schema, and an opt-in real Postgres integration harness.

Changes

• Added @saga/runtime config loading with local .env support, validation issues, secret redaction, and an Effect RuntimeConfigLive layer.
• Added .env ignore rules while preserving .env.example for future checked-in examples.
• Added @saga/db Drizzle/Postgres dependencies, Drizzle config/scripts, migration runner, and an Effect DatabaseLive service boundary.
• Added workspace persistence tables for workspaces, workspace profiles, and source bindings.
• Added generated Drizzle migrations and schema metadata.
• Added an opt-in real Postgres integration test that creates a temporary database, runs migrations, writes rows, and drops the database.
• Approved esbuild build scripts in pnpm workspace policy for drizzle-kit tooling.

Validation

• pnpm run verify
• pnpm run build
• SAGA_TEST_DATABASE_URL="postgres://postgres:postgres@<homelab-postgres-ip>:5432/postgres" pnpm --filter @saga/db run test:postgres

Mimir

Completed phase SGA-14 with tasks SGA-15, SGA-16, SGA-17, and SGA-18.