chore(sitemap): finalize OMISSION_MODE_LEAVE_IN_PLACE cleanup + E2E for canonical import

🤖 Filed by mflesher's AI agent as a follow-up split out of #767.

The canonical resolved-plan refactor of the sitemap CSV import shipped in #808 (single plan feeding preview/validate/token/apply, shared capacity helper). Two items were explicitly deferred out of that PR and are tracked here:

- **Proto-enum cleanup**: remove/finalize the deferred `OMISSION_MODE_LEAVE_IN_PLACE` handling in the omission-mode enum now that the canonical plan applies remove-omitted semantics.
- **E2E coverage**: add end-to-end coverage for the canonical import flow (preview → validate → apply), including the omission-mode paths.

Refs: #767, #808.

Start by reading the canonical sitemap CSV import refactor in #808 and trace its preview, validate, and apply flow. Inspect the omission-mode enum and the deferred OMISSION_MODE_LEAVE_IN_PLACE handling, then add E2E coverage for the canonical flow and its omission-mode paths. Done means the deferred cleanup is finalized and all three stages are covered end to end.

Written by the indexing model from the issue text.