v0.4.0: Move Go implementation into subtree (non-breaking)

[tazarov]

Goal

Move root implementation files into a dedicated Go subtree while preserving the current public import path (github.com/amikos-tech/chroma-go-local) and API behavior.

Scope

• Define target package layout under a go/ subtree.
• Move concrete implementation files from repo root into new package locations.
• Keep package boundaries explicit (public facade, internal impl, ffi/loading, etc.).

Proposed Layout (draft)

• go/internal/runtime/...
• go/internal/library/...
• go/internal/config/...
• go/internal/errors/...
• root package remains thin facade only.

Acceptance Criteria

• Core implementation no longer lives in repo root.
• No public API behavior change.
• make test and make lint pass.
• No new circular dependencies.

[tazarov]

Tracked by umbrella #45 (v0.4.0 root-cleanup plan).

[tazarov]

Completed: Go implementation moved to internal/runtime/ and internal/library/. Root package is now a thin facade with type aliases. make test and make lint pass.