ci(sdp): switch reviewer model to claude-opus-5.5

[openlifu-agent]

Summary

Copilot no longer offers claude-opus-4.7, so the SDP Independent AI
Review step fails on every PR:

Error: Model "claude-opus-4.7" from --model flag is not available.

This PR switches pr-sdp-review.yml to claude-opus-5.5.

Refs #52
Refs OpenwaterHealth/openlifu-diathermy-application#57

Test plan (executed)

I put the identical one-line change on an operator-interface branch
and dispatched the workflow from it against
OpenwaterHealth/openlifu-operator-interface#130 (run 38331998092). The
run succeeded, and openlifu-agent posted a full review titled
"Reviewer: GitHub Copilot / Claude Opus 5.5".

This PR can't get an SDP review of its own. pull_request_target
runs the workflow file from the base branch, which still names the
retired model, until this merges.

Self-review

• Assumption I'm making: claude-opus-5.5 is the Copilot CLI ID
  and the org policy enables it for openlifu-agent. The validation
  run confirms both.
• Alternative considered: Reading the model from an org variable
  (vars.SDP_REVIEW_MODEL) so the next model change needs no PRs. I
  didn't take it here because the reviewer prompt also names the model
  in the review record, so the two would drift. It can be revisited in
  #57.
• New dependency introduced: none.
• Test coverage: workflow change only. It was validated with the
  dispatch run above.
• Breaking change: none.
• Tech-debt filed: none.