Use reusable GitHub Actions workflows

[kitsuyui]

Summary

• Replace duplicate workflow bodies with thin callers to kitsuyui/gh-actions-workflows.
• Keep repository-local triggers and permissions in the caller workflow files.

Changed files

• .github/workflows/gitignore-in.yml
• .github/workflows/spellcheck.yml

Validation

• actionlint -color=false was run against the rewritten workflow files from the hub workspace.
• git diff --check passed for this repository before commit.

[github-actions]

🎉 Happy commit!

• dogfooding: PR Use reusable GitHub Actions workflows #820 always celebrates.
• dogfooding: 6b9ec8f321d459001e4af606fc16d97206189877 always celebrates.

[github-actions]

Code Metrics Report

	main (3e38265)	#820 (bdbfce8)	+/-
Coverage	99.0%	99.0%	0.0%
Code to Test Ratio	1:0.5	1:0.5	0.0
Test Execution Time	1s	2s	+1s

Details

  |                     | main (3e38265) | #820 (bdbfce8) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          99.0% |          99.0% | 0.0% |
  |   Files             |              5 |              5 |    0 |
  |   Lines             |            107 |            107 |    0 |
  |   Covered           |            106 |            106 |    0 |
  | Code to Test Ratio  |          1:0.5 |          1:0.5 |  0.0 |
  |   Code              |           1386 |           1386 |    0 |
  |   Test              |            758 |            758 |    0 |
- | Test Execution Time |             1s |             2s |  +1s |

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Reported by octocov