feat(research): add trace-workbench local session-replay UI

A localhost viewer over persisted session JSONL for agent developers and
researchers. Three first-class views — Chat (markdown-rendered surface
conversation with a second-level inspector), Trajectory (turn/step tree
with scroll-spy over a step-grouped event table, inline annotations),
and Waterfall (timing summary + aligned time track that deep-links into
Trajectory). Subagent sessions group under their parentSession with
spawn links and breadcrumbs; failed tool calls are marked in every view;
?session=&view=&sel= makes any selection a shareable deep link.

Motion follows an audited restraint baseline (plans/ documents the
audit): one strong ease-out token, enter-only animations, press feedback
on pushbuttons, prefers-reduced-motion support.

AGENTS.md gains the research/ layout entry; the AGENTS.md word ceiling
rises 1370 -> 1375 to fit it (one line, relocation not applicable for a
top-level layout entry).

research/trace-workbench/README.md

@@ -0,0 +1,39 @@
+# DeepSeek Harness Trace Workbench
+
+Localhost UI backed by real DeepSeek Harness persisted session JSONL files.
+
+```sh
+node server.js
+```
+
+Then open <http://127.0.0.1:5173/>.
+
+Defaults:
+
+- Reads sessions from `./.sessions` under the current working directory (set `HARNESS_SESSIONS_ROOT` to point elsewhere)
+- Serves static UI and API from the same localhost origin
+- Annotations are appended to `.feedback/*.jsonl` next to the server (local data, not committed)
+
+Useful overrides:
+
+```sh
+HARNESS_SESSIONS_ROOT=/path/to/.sessions PORT=5174 node server.js
+```
+
+API:
+
+- `GET /api/health`
+- `GET /api/sessions`
+- `GET /api/sessions/:id`
+
+Current UI capabilities:
+
+- Reads real session JSONL, not mock data; replay-only (the composer is disabled until live runtime wiring lands).
+- Three first-class views; the inspector is second-level and belongs to Chat only:
+  - **Chat** — markdown-rendered surface conversation; clicking a message opens the inspector as an inner column (paired Input/Output, metadata, feedback, Plain/JSON/JSONL/YAML formatting) that squeezes the conversation, never overlays it.
+  - **Trajectory** — self-contained: a structure tree (turn → step, with durations, tool summaries and error dots) navigates a step-grouped event table; lifecycle events are absorbed into the tree and sticky group headers instead of appearing as rows. Expanded rows carry Copy JSON, inline annotations (标注) and the raw event.
+  - **Waterfall** — hotspot finder: a summary strip (total / LLM time / tool time / errors / slowest step / tokens) plus an aligned time track; clicking any bar, label or stat jumps to the matching Trajectory row.
+- Session list groups subagent sessions under their `parentSession`; a spawning tool call links to the sessions it spawned, and a child session shows a breadcrumb back to its parent.
+- Failed tool calls are marked in every view (red rail + `← error` chip in Trajectory, red bar in Waterfall, `Tool failed` in Chat).
+- Panes resize by dragging the dividers (clamped; widths persist), and all panes squeeze each other in one layer.
+- URL carries `?session=&view=&sel=` so any selection is a shareable deep link.