fix: retry _gh_get once on transient network failure before giving up

[essentialbit]

Summary

• community._gh_get() returned None on any request exception or non-200 response, with no distinction from a legitimate final empty page.
• Every paginating caller (pr_merge_planner.py, and scripts/audit_stale_branches.py in open PR chore: add read-only stale/orphan branch audit script #583) uses if not batch: break — a transient network hiccup mid-pagination silently truncated results instead of erroring. Hit once for real: audit_stale_branches.py's branch fetch silently dropped from 172 to 100 branches on page 2 before a manual re-run recovered it.
• Adds one retry (2s delay) inside _gh_get itself before giving up, closing the gap for every caller of this shared helper without touching each script's own loop shape.

Test plan

• from community import _gh_get imports cleanly
• from main import * still imports cleanly (shared module surface)
• Mocked-requests scratch verification (3 cases, all pass):
  • transient failure on attempt 1, success on attempt 2 → returns real data, exactly 2 attempts
  • persistent failure both attempts → returns None, exactly 2 attempts (bounded, no infinite retry)
  • legitimate empty page (200, []) on first try → returns [] immediately, no spurious retry
• No tests/ directory exists in this repo yet (consistent with other recent PRs)

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