fix(pr): enforce serial merge order in PR completion queue

[jafreck]

Problem

When cadre resumes with multiple existing open PRs (e.g., Wave 0: #16, #18, #19, #22), they all attempt to merge into main in parallel. Since each merge changes main, all the other concurrent PRs immediately conflict with the updated base — even after conflict resolution, the retry sees stale dirty state from the other parallel merges.

Root Cause

The completion queue used p-limit(maxParallelIssues) (typically 8), allowing multiple PRs to merge concurrently. But merging into a shared base branch is inherently sequential: each merge changes the base, and the next PR must see that updated state.

Fix

• Force completion queue to serial execution (p-limit(1)) — this is always correct for merging into a shared base branch
• Remove the configurable concurrency constructor parameter since it should always be 1
• PRs still respect DAG dependency ordering via ensureExecution(), now they also respect the implicit ordering that merging into main requires

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.23%. Comparing base (444eb3c) to head (2e7f853).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
- Coverage   93.23%   93.23%   -0.01%     
==========================================
  Files         193      193              
  Lines       19573    19571       -2     
  Branches     3087     3086       -1     
==========================================
- Hits        18249    18247       -2     
  Misses       1290     1290              
  Partials       34       34              

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