feat: visual eval trend dashboard (agent-strace dashboard --trend) (#74)

Extends dashboard with --trend mode: reads per-session eval scores from
eval.json, computes error/retry/cost time-series, and renders a
self-contained HTML report with inline SVG charts.

- TrendPoint, TrendReport, EvalScorePoint data structures
- build_trend_report(): extracts metrics from events + eval.json
- format_trend_terminal(): terminal summary with pass-rate deltas
- render_html_trend(): self-contained HTML, no CDN, no JS libraries,
  inline SVG sparklines with annotation markers
- save_annotation() / load_annotations(): persist timeline markers
- CLI: --trend, --since Nd, --html FILE flags on dashboard subcommand
- dashboard annotate --date --note subcommand
- 25 new tests covering SVG rendering, annotation storage, trend
  builder, terminal formatting, and HTML output

Closes #68

Co-authored-by: Ona <no-reply@ona.com>

Author: Siddhant-K-code

src/agent_trace/cli.py

@@ -592,6 +592,16 @@ def build_parser() -> argparse.ArgumentParser:
     p_dash.add_argument("--limit", type=int, default=50, help="max sessions to show (default: 50)")
     p_dash.add_argument("--agent", default="", help="filter by agent name")
     p_dash.add_argument("--output", "-o", help="write HTML dashboard to this file")
+    p_dash.add_argument("--trend", action="store_true",
+                        help="show quality and behavioral metrics over time")
+    p_dash.add_argument("--since", metavar="Nd",
+                        help="limit trend to sessions from the last N days (e.g. 30d)")
+    p_dash.add_argument("--html", metavar="FILE",
+                        help="write self-contained HTML trend report to FILE")
+    dash_sub = p_dash.add_subparsers(dest="dash_command")
+    p_dash_ann = dash_sub.add_parser("annotate", help="add a timeline annotation")
+    p_dash_ann.add_argument("--date", required=True, help="date in YYYY-MM-DD format")
+    p_dash_ann.add_argument("--note", required=True, help="annotation text")
 
     # annotate
     p_ann = sub.add_parser("annotate", help="attach notes, labels, and bookmarks to trace events")