feat: read schema from file lists and migration directories

[omer-cengel]

Summary

sql[].schema now accepts a list of files and directories in addition to a single file, so a query group can read the schema from an existing Flyway-style migration directory instead of a hand-maintained snapshot. Directory contents are read in a deterministic order that matches Flyway versioned migrations, each file is parsed on its own, and failures name the file or directory responsible.

Changes

• Configuration: sql[].schema is either one path or a non-empty ordered list of paths, each resolved against the configuration file's directory. The single-path form and its diagnostics are unchanged; an empty list and a blank or wrong-typed listed path are rejected with the element named.
• Source loading: a listed file contributes itself; a directory contributes its regular .sql files (case-sensitive, non-recursive), expanded in place in the list. Versioned V<version>__<description>.sql files come first, compared part by part as numbers (V1, V1_1, V2, V10), followed by the remaining .sql files by name; U<version>__... undo files are ignored. The order does not depend on filesystem listing order.
• Diagnostics: a directory with duplicate migration versions, a directory with no .sql files, and an unreadable directory each fail with a one-line message naming the directory, before any output is written.
• Compilation: every schema file is parsed separately and its tables are appended in file order to the entry's schema; a schema syntax or statement error names the file that contains it. A migration directory generates the same repository source as the equivalent single snapshot.
• Documentation: the configuration reference describes the accepted forms, the directory order, and the failures; the PostgreSQL guide's schema input section links to it.

Scope and non-goals

• Only CREATE TABLE statements are accepted, as before; no new statements or column types are supported, and a table defined in more than one file is not merged.
• ALTER/DROP schema evolution, recursive directory scanning, glob patterns, Flyway placeholders, configurable file-name prefixes, and repeatable-migration semantics are not supported.
• The queries field is unchanged.