docs: replace Jaeger screenshot TODO in README observability section

[constk]

Problem

The observability section in README.md (around lines 83–94) still has an outstanding TODO where a Jaeger trace screenshot should go. The rest of the observability story (OTel spans, semconv keys, agent_span(...) usage) is well-told, so this is the one visible loose end in that section.

Proposed solution

• Bring up the local stack, exercise /api/v1/echo (or another instrumented route), and capture a clean Jaeger trace screenshot showing the span hierarchy and a few attribute keys.
• Drop the screenshot into a stable path (e.g. docs/images/jaeger-trace.png).
• Replace the TODO block with the image and a one-line caption.

Acceptance criteria

• No remaining TODO in the observability section of README.md
• Screenshot lives under docs/images/ (or equivalent) and is referenced relatively
• Caption names the route being traced and one or two key span attributes shown

Priority rationale

Single visible loose end in an otherwise polished section.

[constk]

Closed via #105 (merged to develop as commit 8938eb7). The README scaffold is in; PNG capture is the one remaining follow-up (HTML-commented recipe at the new section in README.md).