Build PRs targeting main, recognized GitHub Stacks, or other tracked open PRs

[alexreinking]

Summary

• Previously, the scheduler only built PRs whose base branch was literally main, so a PR whose base is another PR's branch (a stacked PR, e.g. [Target] Keep all implied features in canonical representation Halide#9253 based on #9251) was silently never scheduled.
• Gating moves into SafeGitHubEventHandler.handle_pull_request, where the full PR payload is available. A PR is now built if:
  • its base is main (unchanged), or
  • it's part of a recognized GitHub Stack (the stack payload field) — confirmed by inspecting the actual refs/pull/*/merge commit parents on [Target] Replace public fields with getters/setters Halide#9249-#9253 that this ref is chained through every lower PR's own merge ref, so building it already transitively tests against stack.base.ref (main), or
  • its base is otherwise the head of another open PR in the same repo (an informal/pre-Stacks-feature chain, e.g. a long-lived shared integration branch).
• PRs against release/N.x branches are still skipped (unchanged), and anything else (an untracked, possibly short-lived branch) is now explicitly skipped to avoid wasting CI on builds that can be invalidated without warning.

Fixes #148

Test plan

• ruff check, ty check pass (also verified via pre-commit hooks on commit)
• Manually verify via webhook redelivery that a real stacked PR (e.g. [Target] Keep all implied features in canonical representation Halide#9253) now triggers a build
• Manually verify a PR based on an untracked scratch branch is still skipped

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