CI: Wire vally eval invocation to a curated medium-size suite via .vally.yaml

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## Context

Follow-up to [#1912](https://github.com/microsoft/GitHub-Copilot-for-Azure/pull/1912), which migrated Waza eval references to Vally. That PR deliberately wired `.github/workflows/eval.yml` to a single hardcoded `--eval-spec` path to keep scope small and get CI green.

## Goal

Replace the single `--eval-spec` flag with `--suite ` driven by `.vally.yaml` suite config, so CI runs a curated set of eval specs (not just one).

## Target

- **Inner loop (local dev)**: `vally-cli eval --eval-spec ` — fast iteration on a single spec.
- **Medium loop (CI)**: `vally-cli eval --suite pr` — curated set targeting 5–10 minute wall-clock runtime.
- **Outer loop (nightly / manual)**: `vally-cli eval --suite full` or individual dispatches — full coverage.

## Prereqs

- Tag hygiene pass (completed in #1912): `cost: free` now reflects reality, so suite filters behave correctly.
- Define `pr` and `full` suites in `.vally.yaml` with the right tag filters.

## Acceptance

- `.vally.yaml` defines `pr` and `full` suites with explicit tag filters
- `.github/workflows/eval.yml` Run evaluations step uses `--suite pr`
- CI wall-clock time for the `pr` suite is 5–10 minutes
- Results artifact still uploads at the standard path

## Related

- PR #1912 — initial Waza migration groundwork
- Thread: https://github.com/microsoft/GitHub-Copilot-for-Azure/pull/1912#discussion_r3096070310 (jongio's original comment)

Start by comparing .github/workflows/eval.yml with the migration context in PR #1912, then inspect the existing or expected .vally.yaml configuration. Define the pr and full suites with explicit tag filters and update the Run evaluations step to use --suite pr. Confirm the results artifact remains at its standard path and that the pr suite runs in 5–10 minutes.

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