Add a trace JSONL pretty-printer CLI helper

[madeinplutofabio]

Summary

Add a small CLI tool that takes a per-call NCP trace file (<trace_id>.jsonl) and prints a human-readable timeline. Raw JSONL is hard to skim when debugging a graph run; a pretty-printer is a frequent adopter pain point.

Why this is a good first issue

• Self-contained: one new directory under tools/ or a single script.
• Either Python (stdlib only) or Rust is acceptable. Pick the language you are comfortable with.
• Acceptance criteria are concrete and testable.
• No protocol-level or runtime-internal knowledge required. Just JSON parsing and formatting.

Acceptance criteria

• New tool at tools/trace-pretty/ (Rust binary) OR tools/trace_pretty.py (Python stdlib).
• Takes a path to a .jsonl trace file as positional arg.
• Reads the file, parses each line as JSON, prints a human-readable timeline. Suggested columns: step | node_id | brick_id | result_type | latency_ms.
• Prints a summary footer: total steps, total latency, count by result_type.
• Exits 0 on success, non-zero on file-not-found or malformed line.
• README in the tool's directory explains the trace format briefly plus shows example output against a real trace from examples/graphs/echo-pipeline/.

Out of scope

• Real-time tailing (tail -f mode).
• HTML or web rendering.
• Cross-trace aggregation.

Where to read

• runtime/src/trace.rs for the JSONL record shapes (runtime_info, invoke).
• spec/ncp-v0.2.3.md §11 (Tracing) for the canonical schema.
• A real example trace: run ncp-mcp-server with --trace-dir and perform a tools/call via examples/mcp/ci_smoke.py or the manual smoke recipe in examples/mcp/echo-pipeline-mcp-smoke.md.

[beanscg]

Bean Labs took a public-source pass on this issue.

The narrowest useful next step looks like: Propose a docs/reproduction checklist that turns the report into one copy-pasteable maintainer verification path.

Public evidence checked:

• issue state OPEN; repo public by GitHub issue search; assignees=0; comments=0
• title: Add a trace JSONL pretty-printer CLI helper
• license: Apache-2.0 (detected from public raw LICENSE/COPYING file)

A good acceptance check would be a narrow public change that directly exercises the issue's reported boundary.

No PR opened from this pass.