fix: comma-separated --schema/--exclude_schema silently matched nothing

[leochong]

Summary

• --schema public,reporting was documented in the README as supported ("Multiple schemas (comma-separated)"), but the raw comma-joined string was passed straight through to schemainspect's filter_schema(), which compares it for exact equality against each object's schema name — "public,reporting" never equals "public" or "reporting", so the diff was silently empty with no error. Single-schema (--schema public) and no-schema calls were never affected — this only broke 2+ comma-separated names.
• Adds parse_schema_arg() / filter_inspector_schemas() in migra/util.py. filter_inspector_schemas() reuses schemainspect's own PROPS list (the same one its filter_schema() iterates over) instead of hardcoding a second copy, so it can't silently drift out of sync as schemainspect adds new object types.
• Adds _get_inspector() in migra/migra.py, used at all 5 of Migration's inspector-construction call sites (__init__ x2, inspect_from(), inspect_target(), apply()). Single-schema/no-schema calls are passed straight through to the original get_inspector() unchanged — zero behavior change there; only 2+ comma-separated names take the new post-filter path.

Test plan

• New tests in test_migra.py: test_multischema, test_multischema_whitespace_tolerant (surrounding whitespace in the comma list), test_exclude_multischema — each fixture (tests/FIXTURES/multischema/, tests/FIXTURES/exclude_multischema/) includes a third, unlisted schema so the assertions prove real filtering (expected cross-schema changes present, third-schema changes absent), not just "happens to work with one schema"
• These tests exercise both the CLI path (run()) and Migration.apply() directly, via do_fixture_test's existing two-phase structure
• Full suite: 356 passed, 2 skipped, no regressions
• flake8 + black clean

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