Pin spellcheck workflow ref

[kitsuyui]

Summary

• Pin the reusable spellcheck workflow to an immutable commit SHA.
• Keep the workflow behavior unchanged while avoiding a moving branch ref.

Verification

• actionlint .github/workflows/*.yml
• git diff --check origin/main...HEAD
• Confirmed the pinned reusable workflow ref resolves through the GitHub API.

[github-actions]

Code Metrics Report

	main (eba647f)	#1595 (de06e53)	+/-
Coverage	87.2%	87.2%	0.0%
Code to Test Ratio	1:0.5	1:0.5	0.0
Test Execution Time	5s	6s	+1s

Details

  |                     | main (eba647f) | #1595 (de06e53) | +/-  |
  |---------------------|----------------|-----------------|------|
  | Coverage            |          87.2% |           87.2% | 0.0% |
  |   Files             |             11 |              11 |    0 |
  |   Lines             |            141 |             141 |    0 |
  |   Covered           |            123 |             123 |    0 |
  | Code to Test Ratio  |          1:0.5 |           1:0.5 |  0.0 |
  |   Code              |           1119 |            1119 |    0 |
  |   Test              |            614 |             614 |    0 |
- | Test Execution Time |             5s |              6s |  +1s |

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Reported by octocov