feat(traces): add --fields projection flag to traces get and export

[dark-sorceror]

Summary

Fixes #51

traceroot traces get never sent the fields query parameter, so every span came back from the server's default skeleton projection with input: null, output: null, metadata: null — silently, with no error. That's the exact payload someone debugging a trace needs. Meanwhile traces export bundles already contained full I/O (the export endpoint defaults to full server-side), so get and export disagreed about the same trace with nothing in --help or the README explaining why. This PR wires the fields projection parameter (already documented in the vendored openapi.json) through both commands so callers can ask for full or a specific group like io,metadata, and documents the default in --help and the README.

How it works

traces get/export --fields <groups>
  → parsed as a plain string option (onceOption guards against repeats)
    → threaded into ApiClient.getTrace/exportTrace as { fields }
      → appended as ?fields=<groups> on the request URL, omitted entirely when not passed
        → server validates the group list; an unknown group surfaces as a normal CliError from its 400

traces get keeps the server's skeleton default (no per-span input/output/metadata) since it's the fast, common-case view. traces export keeps the server's full default since a bundle written to disk is expected to be complete. --fields overrides either default without changing anything else about the request.

What's included

API client

• src/api/client.ts — new TraceFieldsParams type; getTrace/exportTrace now take an optional params and append ?fields=... to the request URL only when a value is provided

Commands

• src/commands/traces/get.ts — --fields <groups> option (reuses onceOption from traces/list.ts), threaded from RunGetDeps into client.getTrace
• src/commands/traces/export.ts — --fields <groups> option, threaded from ExportDeps into client.exportTrace

Docs

• README.md — documents that traces get defaults to the lightweight skeleton projection and traces export defaults to full, with a --fields full example

Test plan

• 597/597 vitest tests pass; typecheck and lint/format clean
• New unit tests pin that fields appears in the outgoing URL for getTrace/exportTrace, and is absent when omitted (tests/api/client.test.ts)
• New unit tests confirm --fields threads from runGet/runExport through to the client call, and that omitting it passes fields: undefined (tests/commands/traces-get.test.ts, tests/commands/traces.export.test.ts)
• Live e2e against app.traceroot.ai on a 123-span trace:
  • traces get --json with no flag → 0/123 spans with input (confirms the bug and the documented default)
  • --fields full → 120/123 spans with input
  • --fields io,metadata → same result as full
  • --fields bogus → clean CLI error listing the valid groups (no stack trace)
  • traces export --fields skeleton → spans.json written without I/O
  • traces export with no flag (default) → spans.json written with I/O
• --help output for both traces get and traces export documents the default projection and how to override it

---

Summary by cubic

Adds a --fields projection flag to traces get and traces export, wiring it through the API client so you can request full or specific groups like io,metadata. This also fixes missing span I/O in traces get and makes get/export consistent.

• New Features

  • --fields <groups> for traces get and traces export; forwarded to the server as ?fields=....
  • API: getTrace(traceId, { fields }) and exportTrace(traceId, { fields }); only send the query when provided.
  • Defaults: traces get keeps skeleton; traces export keeps full. The flag overrides either.
  • Help text and README updated with examples.

• Bug Fixes

  • traces get now sends fields when requested, fixing silent input/output/metadata: null responses and aligning with export.
  • Tests cover URL query handling and flag threading for both commands.

^{Written for commit 199ce67. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

_{Re-trigger cubic}