[STG-2257] fix(ci): skip Stainless jobs for fork PRs

[shrey150]

Summary

Fork-triggered pull_request workflow runs don't receive repo configuration variables or secrets, so STAINLESS_ORG/STAINLESS_PROJECT arrive empty and the Stainless action dies immediately with Input required and not supplied: project on every external-contributor PR. Seen on #2241 (claimed copy of #2240): the fork PR's failing preview check attached to the shared head SHA, leaving the claimed PR red through no fault of its own.

This guards both jobs on github.event.pull_request.head.repo.full_name == github.repository:

• preview — skips on fork PRs instead of failing. Claimed copies of external PRs run in-repo, so SDK preview coverage is preserved.
• merge — same defect: a directly-merged fork PR would trigger a guaranteed-failing merge build (still no vars/secrets on the closed event). Skipping is strictly better than failing; the claim flow means merged external work is always in-repo anyway.

The full_name comparison is used instead of !head.repo.fork so a deleted fork repo (head.repo == null) skips rather than runs-and-fails.

Linear: STG-2257

E2E Test Matrix

Command / flow	Observed output	Confidence / sufficiency
Failing fork run: actions/runs/27375420406 (triggered by fork PR #2240)	Env dump shows STAINLESS_ORG: and STAINLESS_PROJECT: empty; action exits with Error: Input required and not supplied: project	Proves the root cause is withheld vars on fork runs, not the spec or config — exactly the case the new guard skips
actionlint .github/workflows/stainless.yml on this branch	Exit 0, no findings	Validates workflow schema and the new if: expression syntax
preview check on this PR (in-repo branch)	See checks below — job runs (guard evaluates true for same-repo PRs) and builds SDK previews	Live proof the guard doesn't skip in-repo PRs; the fork-skip path can only be exercised by the next real fork PR

🤖 Generated with Claude Code

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Summary by cubic

Skip Stainless CI jobs for fork-origin PRs to avoid failures from missing repo vars/secrets. Keeps external PRs green while coverage runs on in-repo “claimed” copies. Addresses Linear STG-2257.

• Bug Fixes
  • Skip preview and merge unless github.event.pull_request.head.repo.full_name == github.repository; merge still requires closed+merged into main.
  • Use the full_name check to safely skip deleted forks (head.repo can be null).

^{Written for commit 58c5891. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}