ci: add DCO config to allow remediation commits

[EngHabu]

Summary

• Adds .github/dco.yml configuring the Probot DCO app to allow remediation commits.
• Enables both individual (a contributor signing off their own past commits) and thirdParty (someone else signing off on a contributor's behalf) remediation modes.

Why

The Probot DCO app reads its config from the default branch, so this needs to land on master first before remediation commits on other branches will be honored. The immediate motivation is the v2 → main integration PR (#6583), which has 133 commits across ~20 authors flagged for missing/mismatched sign-offs and cannot be cleanly resolved by git rebase --signoff (the default fix the bot suggests works only when the PR author is the sole contributor).

Behavior

• New PRs to any branch still require Signed-off-by: per commit (default strict check unchanged for normal flow).
• A separate "remediation" commit can now be added to backfill sign-offs for prior commits without rewriting history.

Test plan

• After merge, re-run DCO check on PR Flyte 2: Cut over to Flyte 2 new backend implementation #6583 once a remediation commit is pushed and confirm the bot honors it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.96%. Comparing base (9d5bfd7) to head (bbfe1ef).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7297   +/-   ##
=======================================
  Coverage   56.96%   56.96%           
=======================================
  Files         931      931           
  Lines       58272    58272           
=======================================
  Hits        33195    33195           
  Misses      22018    22018           
  Partials     3059     3059           

Flag	Coverage Δ
unittests-datacatalog	53.51% <ø> (ø)
unittests-flyteadmin	53.15% <ø> (ø)
unittests-flytecopilot	43.06% <ø> (ø)
unittests-flytectl	64.09% <ø> (ø)
unittests-flyteidl	75.71% <ø> (ø)
unittests-flyteplugins	60.19% <ø> (ø)
unittests-flytepropeller	53.71% <ø> (ø)
unittests-flytestdlib	62.62% <ø> (ø)

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