Add perf-gate baseline seeding workflow

[Neil4561]

Summary

• Adds the manual performance regression gate baseline seeding workflow to develop.
• The workflow is manual only (workflow_dispatch, with no push or pull_request triggers).
• It is dispatched against the perf gate POC branch, where the seeding orchestrator and the rest of the gate tooling currently live.

Why this is being made now

GitHub only exposes the "Run workflow" control for a workflow_dispatch workflow when the workflow file exists on the default branch (develop). The perf gate POC is now ready to collect real baseline seed data, so the seeding step needs to be dispatchable. This PR registers the manual workflow on develop purely to unblock that.

No other gate code is added here. The gate tooling still lives on the POC branch, and the workflow is intended to be dispatched against that branch (you select it under "Use workflow from"), so the run uses that branch's checkout for the orchestrator and the prebuilt CI image.

What it does when run

• For each requested branch, re-runs the benchmark at recent commits inside the published CI image.
• Source mounts each historical commit so the container tags the real commit SHA on every sample.
• Computes FPS with the same oracle path the live gate uses, then appends the samples to the perf-baselines branch, each stamped with its own target branch so merge-base isolation has a populated, branch-correct baseline.

Safety and scope

• Manual only. It never triggers automatically.
• Fork only. It writes to this fork's perf-baselines branch and does not touch the upstream repository.

Test plan

• After merge, confirm the workflow appears under Actions on develop with a "Run workflow" button.
• Dispatch it against the POC branch with a small scope (one branch, one commit, one sample) and confirm a sample lands on perf-baselines.