feat: add --ignore-schema to exclude schemas from the diff

[Apakottur]

What

A new repeatable flag --ignore-schema (library param ignore_schemas: Sequence[str]) that excludes named schemas from the diff entirely — their tables and every other object, and the CREATE/DROP of the schema itself, are ignored. This includes object kinds pgmig cannot otherwise process (e.g. a rule or an invalid index in an ignored schema no longer blocks the run).

Details

Implemented at the introspection boundary, so the diff layer is untouched — ignored objects simply never exist in the model:

• Guards (unsupported, invalid_indexes, matview_dependencies) read the ignore set from the introspection context and skip findings in ignored schemas, so an unsupported/invalid object in one does not raise.
• introspect_db prunes the assembled DbIntrospectionResult of every ignored schema and all references to it, after the loaders run (they share queries with cross-schema dependency edges, so pruning is applied once to the finished model): schema_by_name, extensions installed into those schemas, the view / matview / view-column / composite-type / enum-column dependency maps (by key and by value), and schema-scoped default-privilege rules. Empty ignore set → no-op early return.
• Wired through agenerate/generate and the CLI.

source	target	--ignore-schema ext
—	ext schema + its objects	(empty diff)
ext schema + its objects	—	(empty diff)
ext.t differs; public.t new		only CREATE TABLE public.t
ext has a rule / invalid index		no PgmigUnsupportedError

Notes

• The value-side pruning of dependency maps matters for correctness, not just tidiness: e.g. a kept enum whose value list changes must not drag an ignored-schema column into its rewrite. (Topological ordering already tolerates stray edges, but the enum-rewrite path walks the dependency list directly.)
• Design choice: applied as a post-introspection model prune rather than editing ~15 introspection SQL files — one contained change, and the diff generators need no awareness of the flag.
• Verified: 11 new tests (create/drop/drift suppression, other-schemas-still-diffed, multiple schemas, all three guards, a control without the flag, plus a direct unit test of the prune covering every structure) — each migration applied and confirmed to converge. 573 tests across the API/introspect/diff/CLI suites green; mypy + ty + ruff + cspell clean; new code at 100% branch coverage. README flags table updated.