Trace: verify Pi post-push session link

Problem

The Pi post-push session-link matrix cell is Unknown. The local E2E suite has no control-plane session URL to assert.

Verify the user-facing URL after a Pi shell-triggered git push of a traced commit, once an upload can be confirmed.

In a disposable authorized control-plane project, run a Pi session that creates and pushes one traced commit.
Capture the matching session URL exactly once in Pi TUI, and through stderr in headless mode without corrupting JSONL stdout.
Resolve the URL and assert it identifies the uploaded session ID for that commit.
Document the case where upload fails: do not claim a successful session link.
Update only the post-push session-link Pi matrix cell after the live assertion passes.

Credentials-warning UI checks or the welcome banner.
Cryptographic verification of the uploaded attestation; tracked by Trace: verify Pi control-plane attestation at push #3718.

Depends on an authorized control-plane upload (#3718). Split from #3718; baseline Pi support #3704.