fix(remediation): avoid read-after-write lag on freshly created fix branches

[franzramadhan]

Summary

• CommitFiles re-fetched the branch ref via GetRef immediately after EnsureBranch created it, and GitHub's Data API can 404 on that immediate re-read (read-after-write lag), causing remediation to fail with fetching branch ... 404 Not Found.
• EnsureBranch now returns the branch's resulting head SHA (existing ref's SHA, or fromSHA when just created) and CommitFiles takes it as a parameter instead of re-fetching, removing the redundant, consistency-sensitive call.
• Observed in production logs: actions-pinned remediation on reearth/reearth-cms failed twice with this error while npm-ci-required (whose branch already existed from a prior run) succeeded.

Test plan

• go build ./...
• go vet ./...
• go test ./... (all packages pass)
• Updated internal/github/write_test.go and internal/remediation/engine_full_test.go for the new EnsureBranch/CommitFiles signatures

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.06%. Comparing base (b185063) to head (604fbbb).

Files with missing lines	Patch %	Lines
internal/remediation/engine.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
- Coverage   78.08%   78.06%   -0.03%     
==========================================
  Files          41       41              
  Lines        3153     3150       -3     
==========================================
- Hits         2462     2459       -3     
  Misses        549      549              
  Partials      142      142              

Flag	Coverage Δ
unittests	78.06% <90.00%> (-0.03%)	⬇️

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