Read Neo4j connection options from environment variables

[rahlk]

Node labels and relationship types are independent Neo4j namespaces; unprefixed
names from different language backends collide in a shared database (for example,
MERGE on :Application, the shared :Symbol merge label, or the CALLS type). Prefix
every node label with Py and every relationship type with PY_; prefix
constraint and index names with py_.

Scope:

• catalog, schema (DDL including constraint/index names), project, both writers, tests
• shared MERGE label :Symbol -> :PySymbol
• regenerate schema.neo4j.json
• SCHEMA_VERSION remains 1.0.0 (no released consumer of the unprefixed form)