bug(exec): {{traceparent}} arg substitution emits all-zero traceparent in non-recording mode

[tobert]

Found while implementing #33.

In otelcli/exec.go, the {{traceparent}} argument substitution uses the function-scoped tp variable:

var tp traceparent.Traceparent
if config.GetIsRecording() {
    tp = otlpclient.TraceparentFromProtobufSpan(span, ...)   // assigned
    injectTraceparent(tp, appendChildEnv)
} else if !config.TraceparentIgnoreEnv {
    passthrough := config.LoadTraceparent()                  // outer tp NOT assigned
    if passthrough.Initialized {
        injectTraceparent(passthrough, appendChildEnv)
    }
}
...
tpArgs[i] = strings.Replace(arg, "{{traceparent}}", tp.Encode(), -1)   // uses outer tp

In non-recording mode the outer tp is never assigned, so tp.Encode() yields an all-zero traceparent (00-000…0-000…0-00). The child's TRACEPARENT env var is correct (it goes through injectTraceparent(passthrough, …)), but the {{traceparent}} arg substitution is not — they disagree.

Repro

TRACEPARENT=00-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa-bbbbbbbbbbbbbbbb-01 \
  otel-cli exec -- echo '{{traceparent}}'
# prints 00-000...0-000...0-00 instead of the passed-through traceparent

This predates #33 (the inner variable was previously also named tp, shadowing the outer); #33 renamed it to passthrough for clarity but preserved the behavior.

Fix sketch

Assign the passthrough traceparent to the outer tp in the non-recording branch so arg substitution and env injection use the same value. Add a functional fixture: non-recording exec ... echo {{traceparent}} with a TRACEPARENT in env should echo the passed-through (lower-cased) traceparent.