feat: add cross-process context propagation

[imran-siddique]

Summary

• propagate standard W3C trace context alongside minimal AgenTrust run, workflow, and upstream-agent metadata
• support synchronous remote-parent continuation and asynchronous span links
• reject unsafe or oversized metadata values and document the trust boundary

Evidence

• python -m pytest -q tests/test_propagation.py (5 passed)
• python -m pytest -q (25 passed, 15 subtests)
• schema, version, and conformance gates pass
• wheel build and fresh-environment smoke install pass

Limits

• mutable string mapping carriers only in this slice
• propagated identity metadata remains untrusted and is not authorization evidence