fix(timeline): report true total_events in robot JSON meta

The robot JSON envelope's meta.total_events field was incorrectly
reporting events.len() (the post-limit count), making it identical
to meta.showing. This defeated the purpose of having both fields.

Changes across the pipeline to fix this:

- collect_events now returns (Vec<TimelineEvent>, usize) where the
  second element is the total event count before truncation
- TimelineResult gains a total_events_before_limit field (serde-skipped)
  so the value flows cleanly from collect through to the renderer
- main.rs passes the real total instead of the events.len() workaround

Additional cleanup in this pass:

- Derive PartialEq/Eq/PartialOrd/Ord on TimelineEventType, replacing
  the hand-rolled event_type_discriminant() function. Variant declaration
  order now defines sort tiebreak, documented in a doc comment.
- Validate --since input with a proper LoreError::Other instead of
  silently treating invalid values as None
- Fix ANSI-aware tag column padding with console::pad_str (colored tags
  like "[merged]" were misaligned because ANSI escapes consumed width)
- Remove dead print_timeline_json and infer_max_depth functions that
  were superseded by print_timeline_json_with_meta

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

src/main.rs

@@ -1416,12 +1416,9 @@ fn handle_timeline(
     let result = run_timeline(&config, &params)?;
 
     if robot_mode {
-        // total_events_before_limit: the result already has events truncated,
-        // but we can compute it from the pipeline if needed. For now, use events.len()
-        // since collect_events already applied the limit internally.
         print_timeline_json_with_meta(
             &result,
-            result.events.len(),
+            result.total_events_before_limit,
             params.depth,
             params.expand_mentions,
         );