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/mr_discussions.rs

@@ -269,14 +269,14 @@ pub async fn ingest_mr_discussions(
     }
 
     info!(
-        mrs_processed = mrs.len(),
-        discussions_fetched = total_result.discussions_fetched,
-        discussions_upserted = total_result.discussions_upserted,
-        notes_upserted = total_result.notes_upserted,
-        notes_skipped = total_result.notes_skipped_bad_timestamp,
-        diffnotes = total_result.diffnotes_count,
-        pagination_succeeded = total_result.pagination_succeeded,
-        "MR discussion ingestion complete"
+        summary = crate::ingestion::nonzero_summary(&[
+            ("MRs", mrs.len()),
+            ("discussions", total_result.discussions_fetched),
+            ("notes", total_result.notes_upserted),
+            ("skipped", total_result.notes_skipped_bad_timestamp),
+            ("diffnotes", total_result.diffnotes_count),
+        ]),
+        "MR discussion ingestion"
     );
 
     Ok(total_result)