feat: offer a self-contained generation mode without a sqlode runtime dependency

[nao1215]

Roadmap: #1

Problem

Generated code still requires sqlode itself as a runtime dependency because every output imports sqlode/runtime.
That makes the generated package less self-contained than sqlc users typically expect.

Evidence

• README.md explicitly says: "sqlode must be a dependency (not just a dev-dependency) because the generated code imports sqlode/runtime"
• Generated params.gleam / queries.gleam modules import sqlode/runtime
• This means evaluating sqlode is a two-step commitment: install the generator and also keep the generator package in the target project

Expected

• Offer an optional self-contained generation mode that vendors the minimal runtime helpers into the output package
• Keep the shared sqlode/runtime path if desired, but do not make it the only story
• Document the tradeoff clearly (smaller shared dependency vs. self-contained generated code)

Why this matters

For public adoption, especially from sqlc users, generated code that stands on its own is much easier to evaluate, review, and vendor into existing projects.