support default_tenant_id env-var

[vijayfractl]

An explanation for the strange behavior in dev:

The dev database has old data. When the tenant ID was changed from 00000-* to 12345-* (commit fd599e3), existing records in the dev DB still have tenant = '00000000-...'. Every query
filters by tenant:

  WHERE __tenant__ = '12345678-1234-1234-1234-123456789abc' AND ...                                                                                                                           

So it finds zero rows — all existing dev data is invisible. Production likely got a fresh database after the change, or had a proper migration, so all its records were created with 12345-* from the start.

The "fix" of setting AGENTLANG_DEFAULT_TENANT_ID=00000-* in dev is really just a workaround to match the stale data. The real fix option would be one of:

1. Migrate dev data: UPDATE <tables> SET __tenant__ = '12345678-1234-1234-1234-123456789abc' WHERE __tenant__ = '00000000-0000-0000-0000-000000000000'
2. Recreate the dev database from scratch
3. Keep the env var approach as a permanent escape hatch (what I just implemented)

[vijayfractl]

the fix is redundant