ci(regression): trigger on PR edited events for Graphite base changes

[miguel-heygen]

What

Add edited to the regression workflow's pull-request event types.

This is Wave 0: the operational prerequisite for the Studio timeline landing-wave migration.

Why

Graphite can change a PR base without changing its head SHA. Regression must create fresh evidence for the exact head/base pair before each family merges.

Prerequisite: merged PR #2558 (735128a61a152267a00b99defbd290f489482cbc).

Landing order: Wave 0 before Families A-G.

How

Mirror the existing CI workflow trigger list. No path filters, jobs, or regression behavior change.

Test plan

• YAML parsed and the exact pull-request event list asserted
• bunx oxfmt --check .github/workflows/regression.yml
• Pre-commit hooks passed
• Live base-only edit retriggers regression after this workflow lands
• Unit tests added/updated (not applicable: workflow-only change)
• Manual testing performed (live event validation follows merge)
• Documentation updated (not applicable)

[miguel-heygen]

• ci(regression): trigger on PR edited events for Graphite base changes #2659 👈 (View in Graphite)
• main

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