fix(submit): do not mistake GitHub 5xx errors for a missing PR

[amustafa]

What

sr submit no longer offers to create PRs that already exist when GitHub's API has a bad moment.

Why

The PR survey ran gh pr view <branch> and treated any exit code 1 as "no PR exists". But gh also exits 1 on server failures — during a real GitHub 503 spell, roughly a third of the survey calls failed per pass, each misread as "this branch has no PR", and submit prompted to create PRs for branches deep in review. Saying yes would at best fail on the duplicate; the prompt itself was the bug.

How

• ghPRForBranch now classifies the failure by gh's stderr, not its exit code:
  • no pull requests found → genuinely no PR (nil, nil, as before)
  • HTTP 5xx → transient server failure, retried up to 3 attempts with 1s/2s backoff so one flaky call doesn't poison a whole stack survey
  • anything else (auth, rate limits) → propagates, so submit stops with "failed to check PR status" instead of offering a duplicate
• The gh stub in prepare_test.go now prints the real no pull requests found message — a bare exit 1 is exactly the ambiguity the fix refuses to interpret.
• New tests: unit tests for both stderr classifiers, plus stub-gh tests of the real function (503-then-success retries and returns the PR; no-PR returns nil without error; a 401 surfaces as an error).

🤖 Generated with Claude Code

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