Track SSI import source of truth

[chubes4]

Summary

• Add import run/source artifact provenance and write a static-site-importer-manifest.json into generated themes.
• Record desired generated pages/files/assets and cleanup actions in the import report.
• Clean stale SSI-generated theme files/assets on overwrite and report stale SSI-owned pages without mutating them by default.

Verification

• php -l includes/class-static-site-importer-theme-generator.php
• php -l includes/class-static-site-importer-page-materializer.php
• php -l includes/class-static-site-importer-report-diagnostics.php
• php -l tests/smoke-import-source-of-truth-manifest.php
• php tests/smoke-product-handoff-contract.php
• studio wp eval-file /Users/chubes/Developer/static-site-importer@fix-import-provenance-manifest-clean/tests/smoke-import-source-of-truth-manifest.php --skip-plugins
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Designing and implementing provenance/manifest reconciliation, adding targeted smokes, and drafting this PR description.