Migrate internal architecture docs to docs/internal

docs/internal/output-schemas.md

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+# Audita Output Schemas
+
+## Scope
+
+This document describes the implemented transcript output schema registry.
+
+## Registry Ownership
+
+Output schema registry is owned by `internal/core/outputschema`.
+
+Supported schema keys:
+- `bare-segments`
+- `audita-v1`
+
+## Schemas
+
+`bare-segments`:
+- top-level JSON array of transcript segments.
+
+`audita-v1`:
+- top-level JSON object with:
+  - `schema: "audita-v1"`
+  - `version: "v1"`
+  - `segments: [...]`
+
+Segment fields include `id`, `speaker`, `start`, `end`, `text`, and optional `categories`.
+
+## Validation and Resolution
+
+Config validation and runtime resolution both reject unsupported schema keys.
+
+Unknown schema keys fail with `unsupported output schema` before output emission.
+
+## Output Emission
+
+The selected schema is used by `audita process` when writing:
+- output file (`--output`) or
+- stdout (when no `--output`).
+
+Report metadata records selected `output_schema`.
+
+## Key Tests
+
+- `internal/core/outputschema/registry_test.go`
+- `internal/core/config/config_test.go`
+- `internal/cli/run_test.go`