docs(schema): publish JSON Schema for goten.config.yaml

[dnahilman]

Closes #26.

Summary

Publishes goten.config.schema.json so editors with YAML LSP support give autocomplete, hover docs, and inline validation when editing goten.config.yaml.

Activate with a magic comment at the top of your config:

# yaml-language-server: $schema=https://raw.githubusercontent.com/dnahilman/goten/main/goten.config.schema.json
env_file: .env
database:
  url: ${DATABASE_URL}

Works in VS Code (Red Hat YAML extension), JetBrains GoLand/IntelliJ, Neovim with yaml-language-server, etc.

Why

Until now editing goten.config.yaml was raw text. With the schema in place:

• Field-name typos get flagged (databse: → red squiggle).
• Hover any field for inline docs.
• driver: postgers → "must be one of postgres".
• Autocomplete suggests valid field names.

Steps (matching #26)

• 2.1 Write goten.config.schema.json at repo root — covers env_file, database (url required, driver enum), migrations (core_dir, plugins, table), generate_dir. $id is the raw GitHub URL.
• 2.2 Add magic comment header to examples/basic/goten.config.yaml. Kept full paths in this file because it shares migrations with the repo root via ../../; the shorthand demo lives in the upcoming Quick Start rewrite (docs(wiki): rewrite Quick Start for new projects #27).
• 2.3 README — short pointer near the CLI section.
• 2.4 CHANGELOG [Unreleased] entry.

Design notes

• Schema is editor-only, no CLI-side validator. The Go struct unmarshal already type-checks structural input; JSON Schema adds field-name and enum validation at the editor level. Level 2 (CLI validator) and Level 3 (SchemaStore submission) are explicit non-goals for this PR.
• additionalProperties: false so unknown fields are flagged. This matches the strict Go struct definitions.
• plugins items: schema describes both shorthand (username) and full path (./plugins/username/migrations) as valid — auto-detected by the CLI at runtime (Issue feat(cli): top-level goten init + plugin registry #25's resolvePluginEntry).

Test plan

• make build — all six modules compile (schema does not affect compilation).
• go test ./cmd/goten/... — all green; schema is not consumed by any test.
• Reviewer: open examples/basic/goten.config.yaml in VS Code with Red Hat YAML extension. Confirm:
  • Hover on database: shows description.
  • Type dat → autocomplete suggests database.
  • Change driver: postgres to driver: postgers → red squiggle.

Unblocks

#27 — Quick Start rewrite can now reference the schema URL in its sample config.

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