Docathon: Add workflow to assign user on comment

[nil-is-all]

This workflow assigns a user to an issue when a comment containing '/assigntome' is made, provided the issue has the 'docathon-2026' label.

cc @mergennachin @AlannaBurke

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19294

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❗ 1 Active SEVs

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❌ 2 New Failures, 1 Cancelled Job, 3 Unrelated Failures

As of commit 2ace5ec with merge base 08b2987 ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4d0ab060d16036a2d522570a586f144a383d4c13e2d7551074c41999cae69e41 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 835ae7fd907453eb75bd6db6cdd5222689cdbe3d70c1d8caba8da1230b3391c4 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_static_attention.py::StaticAttentionTest::test_within_transformer
• pull / unittest / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_static_attention.py::StaticAttentionTest::test_within_transformer

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