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/merge_requests.rs

@@ -50,7 +50,7 @@ pub async fn ingest_merge_requests(
     if full_sync {
         reset_sync_cursor(conn, project_id)?;
         reset_discussion_watermarks(conn, project_id)?;
-        info!("Full sync: cursor and discussion watermarks reset");
+        debug!("Full sync: cursor and discussion watermarks reset");
     }
 
     let cursor = get_sync_cursor(conn, project_id)?;
@@ -122,12 +122,14 @@ pub async fn ingest_merge_requests(
     }
 
     info!(
-        fetched = result.fetched,
-        upserted = result.upserted,
-        labels_created = result.labels_created,
-        assignees_linked = result.assignees_linked,
-        reviewers_linked = result.reviewers_linked,
-        "MR ingestion complete"
+        summary = crate::ingestion::nonzero_summary(&[
+            ("fetched", result.fetched),
+            ("upserted", result.upserted),
+            ("labels", result.labels_created),
+            ("assignees", result.assignees_linked),
+            ("reviewers", result.reviewers_linked),
+        ]),
+        "MR ingestion"
     );
 
     Ok(result)