fix(pr): rewrite completion queue to be truly serial

[jafreck]

Problem

PR #339 set p-limit(1) expecting serial execution, but executeItem calls were still spawned concurrently via enqueue() → ensureExecution(). The p-limit only gated the inner merge block, so multiple PRs would still fire their initial merge attempts simultaneously. This caused PR #18 to fail immediately (before #16's conflict resolution even started), and the completion queue appeared hung.

Root Cause

enqueue(#16) → ensureExecution(#16) → executeItem(#16) ─┐
enqueue(#18) → ensureExecution(#18) → executeItem(#18) ─┤ both run concurrently
                                                          └─> p-limit(1) only gates merge portion

Fix

Replaced the entire concurrent task model with a simple sequential drain() loop:

• enqueue() now just collects items into an array
• drain() iterates items one at a time, running the full resolve → wait → retry cycle for each before moving to the next
• Each PR sees the fully updated base branch after the prior merge
• DAG deps are checked via completed/failed sets instead of awaiting in-flight promises
• Idempotent: drain() skips already-processed items

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.22%. Comparing base (730ed0b) to head (49a32bc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   93.23%   93.22%   -0.01%     
==========================================
  Files         193      193              
  Lines       19571    19555      -16     
  Branches     3087     3085       -2     
==========================================
- Hits        18247    18231      -16     
  Misses       1290     1290              
  Partials       34       34              

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