docs: replace Jaeger screenshot TODO with section scaffold (#92)

The observability story in README has one visible loose end: a TODO
block where the Jaeger trace screenshot should go. The rest of the
section reads cleanly, so the TODO sticks out.

Promotes the placeholder to a real subsection ("Jaeger trace") with
the explanatory caption already written: what boots the stack, what
endpoint produces the trace, where to view it, and that span
attributes use only the constant-defined semconv keys from
src/observability/spans.py.

The image itself still needs to be captured. The original capture
recipe is preserved as an HTML comment so it remains discoverable,
and the comment includes the exact one-line markdown to paste in once
docs/images/jaeger-trace.png lands. Hiding the placeholder inside an
HTML comment (rather than a broken-image ref) keeps the rendered
README clean while the PNG is outstanding.

The image-capture step itself is a follow-up — needs the maintainer
to run docker compose locally and take the screenshot.

Closes #92 (capture step tracked separately as a single-line README
edit when the PNG is committed).

Author: constk

README.md

@@ -80,17 +80,23 @@ The scaffold's React page hits `/api/v1/health` on load and renders the version
 
 ![Hello page](docs/images/hello-page.png)
 
+### Jaeger trace (`docker compose up` + `/api/v1/health`)
+
+The full stack — backend, frontend, Jaeger collector — boots with `docker compose up`. Hitting `/api/v1/health` once produces an OpenTelemetry trace exported via OTLP/gRPC; the span hierarchy is visible at <http://localhost:16686> under the `harness-python-react` service, with `agent_span(...)` attributes attached using only the keys constant-defined at the top of [`src/observability/spans.py`](src/observability/spans.py).
+
 <!--
-TODO (#28): one capture left — Jaeger trace.
+Screenshot pending: docs/images/jaeger-trace.png
+
+Capture recipe (run once and commit the PNG to docs/images/):
+  1. docker compose up
+  2. curl http://localhost:8000/api/v1/health
+  3. open http://localhost:16686 -> select service "harness-python-react"
+  4. click the most recent trace
+  5. screenshot the span timeline, save as docs/images/jaeger-trace.png
 
-  docs/images/jaeger-trace.png
-    With the full stack running (`docker compose up`), hit /api/v1/health
-    once, then open http://localhost:16686, select service
-    `harness-python-react`, click the most recent trace, screenshot the
-    span timeline.
+When the PNG is committed, replace this whole comment with:
 
-When the PNG lands in docs/images/, replace this comment with a section
-analogous to "Hello page" above.
+    ![Jaeger trace — span timeline for GET /api/v1/health](docs/images/jaeger-trace.png)
 -->
 
 ## Why a harness