feat(schema,config): pluggable parser registry (#129)

[zeevdr]

Closes #129. Final cross-cutting issue in the Schema Spec v0.1.0 milestone.

Summary

Extracts the schema and config YAML parsers behind a small `Parser` interface + package-level registry. Each spec version is now a single sibling file (e.g. `parser_v1.go`) that declares a parser struct and registers it via `init()`. Adding v2 in the future means landing one new file — the existing v1 path stays untouched, and nothing else in the codebase changes.

Design

```go
type Parser interface {
SpecVersion() string
Parse(data []byte) (*pb.Schema, error)
Marshal(s *pb.Schema) ([]byte, error)
}

func Register(p Parser) // called from each parser's init()
func Dispatch(data []byte) (*pb.Schema, error) // routes by spec_version
func MarshalSchemaAt(s *pb.Schema, version string) (...) // emits in chosen version
func SupportedVersions() []string // surfaced in errors
func LatestVersion() string // default for export
```

(Symmetric in `internal/config/` — `DispatchImport` carries the schema's `fieldTypes` since the config parser needs them for type-aware coercion, and returns a `ParsedImport` struct so the service layer can pick up the YAML's `description:` for audit purposes.)

Layer split

• Layer 1 (per-version, in the parser): YAML shape — required keys, regex patterns, type enums, basic structural lint.
• Layer 2 (version-agnostic, in the service): referential integrity (redirect_to, dependentRequired field existence), prefix-overlap rejection, range sanity, runtime cross-field rules (dependentRequired).

The dispatch point sits between the two — `Dispatch` returns a `*pb.Schema`, then service.go runs the layer-2 checks on that proto value, identical regardless of which version produced it.

Wire surface

• Adds optional `spec_version` field to `ExportSchemaRequest` (proto field 3) and `ExportConfigRequest` (proto field 3). Additive — buf breaking passes.
• Default behavior unchanged: when `spec_version` is omitted, the server emits the highest registered version. With v1 as the only registered parser, that's the same output as before.
• Unknown `spec_version` produces `InvalidArgument` with the supported version list in the error message.

Test coverage

• `internal/schema/parser_test.go` and `internal/config/parser_test.go`:
  • SupportedVersions / LatestVersion
  • Dispatch routing — happy path, missing spec_version, unknown version, malformed YAML
  • MarshalSchemaAt — default (latest), explicit v1, unknown version rejected
  • Round-trip: `Dispatch → MarshalSchemaAt → Dispatch` preserves all fields including `dependentRequired` and `validations`
• Existing `yaml_test.go` suites unchanged — `parser_v1` is a thin wrapper over the same underlying functions.

Docs

• `docs/concepts/meta-schema.md` gains a "Multi-version evolution" section describing the plug-in pattern and the `spec_version` request field.
• `.agents/context/schema-spec.md` updated with the registry shape so future-me / next contributor knows where v2 plugs in.

Test plan

• `make test` passes (full suite, race detector on)
• `make lint` passes
• `make generate docs` regenerated and committed
• No semantic changes to existing v1 parsing — yaml_test.go suites pass without modification

[github-actions]

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