feat(tracing): adopt an edge's trace id without parenting its span

[sylvesterdamgaard]

Load balancers and proxies often inject a traceparent on every request, so their access log carries a trace id that joins the backend's trace, while exporting no span of their own. Continuing that header makes every request span the child of a span that never arrives, and trace backends list those traces as missing their root. We hit this in production behind HAProxy: every app trace in Tempo showed up as "root span not yet received".

What changes

• New traces.continue_incoming_parent (TELEMETRY_TRACES_CONTINUE_INCOMING_PARENT), default true. With the default, nothing changes.
• Set to false, the request adopts the incoming trace id and sampled flag (still subject to trust_incoming_sampling) but starts as the root span. The span id the edge named is kept as a span link instead of a parent, so the edge's log line still joins the trace and the reference isn't silently dropped.
• Tracer::adoptTraceId() sits next to continueFrom(); TraceRequest picks between them.
• AGENTS.md invariant 5 ("children, never detached roots") now names this opt-in exception.

Tests

Three new cases in RequestInstrumentationTest:

• the trace id is adopted with no parent and a link to the edge span
• an unsampled incoming flag is still respected
• with no header, the span is a plain root with no link

composer check is green (Pint, PHPStan, full suite).

Docs

• traces.md has a new "Behind an edge that injects traceparent" section.
• Also updated: the config reference, the AI guidelines, llms.txt and the CHANGELOG (Unreleased/Added).