Stop failing sync over already-sent campaigns still in the repo

A campaign.md is meant to stay around after it's sent as a record of what
went out and when. Treating any non-draft campaign as a hard rejection
forced deleting it just to keep CI green, and there was no way to fail on
resolving a stale list/attachment reference either -- an archived
campaign has nothing left to resolve, so check status before doing any of
that work. Non-draft campaigns now report as a distinct, non-error
"skipped" bucket instead of "rejected".

README.md

@@ -101,11 +101,14 @@ Each file in `attachments:` is uploaded to listmonk's media library under a synt
 
 Every `sync` that actually creates or changes a campaign auto-sends a preview via listmonk's test-send endpoint, to `preview_emails` if set, otherwise `CAMPAIGNS_PREVIEW_EMAIL`. `campaigns test` is for an on-demand re-preview without touching content.
 
+## Sent campaigns stay in the repo
+
+A campaign's `campaign.md` is meant to stick around after it's sent — a record of what went out and when, not something to delete once it's live. Once a campaign is anything other than `draft` in listmonk (`scheduled`/`running`/`paused`/`cancelled`/`finished`), sync leaves it alone entirely — no write, no re-preview, and (as of v0.1.6) no failure either. It's reported as **skipped**, a distinct, non-error bucket from rejected.
+
 ## Sync guardrails
 
 A rejected campaign is usually one of these, all deliberate:
 
-- **Anything other than `draft` in listmonk** (`scheduled`/`running`/`paused`/`cancelled`/`finished`) — sync refuses to touch it once it's live or sent.
 - **A `lists:` name matches zero or more than one listmonk list** — sync never guesses.
 - **An invalid `segment_query`** — listmonk's own error is surfaced verbatim.
 - **Missing `subject`/`from_email`/a target audience, or `type: optin`.**