[codex] add workspace registration and local service

[dbtlr]

Summary

Adds the workspace registration and local service phase: saga init, saga doctor, and foreground saga service run/status commands.

Changes

• Added a source-only CLI bin shim so the local saga command can execute TypeScript packages through tsx during early development.
• Implemented saga init to detect the git project root, register/upsert a Workspace and source binding in Postgres, run migrations, and write .saga.local.json as the gitignored local binding file.
• Added DB workspace registration helpers for workspace/profile/source-binding persistence.
• Implemented saga doctor with Node/pnpm, binding, Postgres connectivity, migration, service, and harness status reporting.
• Added runtime service host/port config via SAGA_SERVICE_HOST and SAGA_SERVICE_PORT.
• Added a foreground service health endpoint and saga service run/status commands.

Validation

• pnpm run verify
• pnpm run build
• pnpm --filter @saga/db run db:test:up
• pnpm --filter @saga/db run test:postgres:local
• pnpm --filter @saga/db run db:test:down
• Manual saga init against local Postgres from a temporary git repo
• Manual saga doctor before/after init
• Manual saga service run and saga service status on a throwaway port

Mimir

Completed phase SGA-19 with tasks SGA-20, SGA-21, and SGA-22.