Align env-based context propagation with OTel env-carriers spec

[tobert]

Tracking issue

Align otel-cli's environment-variable context propagation with the OpenTelemetry env-carriers specification, adopting the shared Go carrier rather than maintaining custom carrier behavior.

Mirrors upstream equinix-labs/otel-cli#390 (filed by @pellared). We're tracking our own decomposition here.

Spec / library

• Spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/env-carriers.md
• Go carrier: https://pkg.go.dev/go.opentelemetry.io/contrib/propagators/envcar

Decisions for our fork

• Adopt envcar (not a from-scratch reimplementation). We already depend directly on go.opentelemetry.io/otel + otel/sdk + otel/semconv, so a contrib propagator is a natural extension, not a foreign dependency.
• envcar@v0.69.0 requires otel core v1.44.0 and Go 1.25 — so the dependency bump is a real, separately-tracked prerequisite.
• TRACESTATE + BAGGAGE share the same composite-propagator plumbing, so they're folded into a single follow-up issue.

Already satisfied in our fork

• exec already injects into a child env copy (otelcli/exec.go builds a separate childEnv; the parent environment is never mutated). The spec's "inject into the child process environment copy" ask needs no change beyond routing it through the carrier.

Sub-issues (do in order)

• chore(deps): bump OpenTelemetry SDK to v1.44+ and Go to 1.25 (prereq for envcar) #32 — SDK + Go version bump (prerequisite)
• feat(propagation): adopt contrib/propagators/envcar for env carrier (TRACEPARENT) #33 — Adopt envcar carrier for TRACEPARENT
• feat(propagation): propagate TRACESTATE and BAGGAGE via standard propagators #34 — Propagate TRACESTATE + BAGGAGE

Out of scope (for now)

• Recording baggage as span attributes / setting tracestate on emitted spans — propagation passthrough only unless we decide otherwise.
• Replacing the otel-cli-specific --tp-* flags or the shell-sourceable tp-carrier file format; those stay and must keep working (clear migration path required).