refactor(ingestion): compact log summaries and quieter shutdown messages

Migrate all ingestion completion logs to use nonzero_summary() for compact,
zero-suppressed output. Before: 8-14 individual key=value structured fields
per completion message. After: a single summary field like
'42 fetched · 3 labels · 12 notes' that only shows non-zero counters.

Also downgrade all 'Shutdown requested...' messages from info! to debug!.
These are emitted on every Ctrl+C and add noise to the partial results
output that immediately follows. They remain visible at -vv for debugging
graceful shutdown behavior.

Affected modules:
- issues.rs: issue ingestion completion
- merge_requests.rs: MR ingestion completion, full-sync cursor reset
- mr_discussions.rs: discussion ingestion completion
- orchestrator.rs: project-level issue and MR completion summaries,
  all shutdown-requested checkpoints across discussion sync, resource
  events drain, closes-issues drain, and MR diffs drain

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ingestion/issues.rs

@@ -109,11 +109,13 @@ pub async fn ingest_issues(
     result.issues_needing_discussion_sync = get_issues_needing_discussion_sync(conn, project_id)?;
 
     info!(
-        fetched = result.fetched,
-        upserted = result.upserted,
-        labels_created = result.labels_created,
-        needing_sync = result.issues_needing_discussion_sync.len(),
-        "Issue ingestion complete"
+        summary = crate::ingestion::nonzero_summary(&[
+            ("fetched", result.fetched),
+            ("upserted", result.upserted),
+            ("labels", result.labels_created),
+            ("needing sync", result.issues_needing_discussion_sync.len()),
+        ]),
+        "Issue ingestion"
     );
 
     Ok(result)