fix: replace invalid commit-docs workflow command

[tsubasakong]

What

• replace the invalid commit-docs workflow step in validate-phase with the supported commit command
• pass the generated VALIDATION.md path via --files so the docs-only commit still targets the intended file
• update the matching plan-phase guidance so both workflows stay consistent

Fixes #968.

Why

• replace the invalid commit-docs workflow step in validate-phase with the supported commit command
• pass the generated VALIDATION.md path via --files so the docs-only commit still targets the intended file
• update the matching plan-phase guidance so both workflows stay consistent

Fixes #968.

Testing

• Tested on macOS
• Tested on Windows
• Tested on Linux

Validation notes:

• git diff --check
• rg -n "commit-docs" get-shit-done/workflows/validate-phase.md get-shit-done/workflows/plan-phase.md

Checklist

• Follows GSD style (no enterprise patterns, no filler)
• Updates CHANGELOG.md for user-facing changes
• No unnecessary dependencies added
• Works on Windows (backslash paths tested)

Breaking Changes

None

Fixes #968