Add public API boundary checks

docs/internal/overview.md

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+# Internal package inventory
+
+Public facades and their contracts are defined in Go declarations and GoDoc.
+The [runtime consumer guide](../consumers/runtime-version.md) covers application
+usage, and the [CLI contract](../cli.md) covers command behavior.
+
+Implemented internal packages:
+
+- `internal/runtimeinfo` holds the injected version value and selects the
+  executable's main-module version when no injected value is present.
+- `internal/versioncontract` names the linker symbol shared with the future
+  release builder.
+- `internal/apicheck` checks compiled exports with `go/types` and checks import
+  direction. Run it with `go run ./internal/apicheck` from the repository root.
+
+The executable currently imports only the runtime facade. Release orchestration,
+process execution, build, Git, and Gitea implementation packages are introduced
+as their responsibilities are implemented.