style-guide: document commit message rules

[ideaship]

What

Fleshes out the empty Commit messages section in the developer style guide (docs/guides/developer-guide/style-guide.md).

Details

• Summarizes the essentials from the OpenStack git commit message guidelines (one logical change per commit, subject line, body) and links to that document as the authoritative source rather than duplicating it.
• Adds a subsection on attributing AI-assisted changes via the Assisted-by trailer, following the Linux kernel coding-assistants guidelines, alongside the required Signed-off-by trailer.

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	5		0	0	0.04s
✅ JSON	jsonlint	4		0	0	0.11s
✅ JSON	prettier	4		0	0	0.42s
✅ JSON	v8r	4		0	0	9.99s
✅ MARKDOWN	markdownlint	156		0	0	2.58s
✅ MARKDOWN	markdown-table-formatter	156		0	0	0.41s
✅ REPOSITORY	betterleaks	yes		no	no	0.76s
✅ REPOSITORY	checkov	yes		no	no	18.47s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	1.85s
✅ REPOSITORY	trufflehog	yes		no	no	3.95s
✅ SPELL	codespell	166		0	0	0.52s
⚠️ SPELL	lychee	166		3	0	27.99s
✅ YAML	prettier	6		0	0	0.37s
✅ YAML	v8r	6		0	0	5.85s
✅ YAML	yamllint	6		0	0	0.5s

Detailed Issues

⚠️ SPELL / lychee - 3 errors

📝 Summary
---------------------
🔍 Total..........879
🔗 Unique.........719
✅ Successful.....810
⏳ Timeouts.........5
🔀 Redirected......49
👻 Excluded........61
❓ Unknown..........0
🚫 Errors...........3
⛔ Unsupported......3

Errors in docs/appendix/security/ossa-2026-001.md
[TIMEOUT] https://bugs.launchpad.net/keystonemiddleware/+bug/2129018 (at 124:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-015.md
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148398 (at 174:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148477 (at 175:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2149775 (at 176:3) | Request timed out

Errors in docs/cloud-in-a-box/index.md
[500] https://cloud-init.io/ (at 468:13) | Rejected status code: 500 Internal Server Error | Followed 1 redirect. Redirects: https://cloud-init.io/ --[302]--> https://cookies.canonical.com/api/v1/cookies/session?return_uri=https%3A%2F%2Fcloud-init.io%2F

Errors in docs/release-notes/index.md
[ERROR] https://release.osism.tech/ (at 11:1) | Connection failed. Check network connectivity and firewall settings

Errors in docs/release-notes/osism-7.md
[TIMEOUT] https://www.openstack.org/software/openstack-bobcat (at 977:38) | Request timed out

Errors in docs/release-notes/osism-8.md
[ERROR] https://docs.openstack.org/releasenotes/skyline-console/2024.1.html (at 242:83) | Connection closed before response completed

Hint: Followed 49 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

Notices

📣 MegaLinter 9.5.0 is out! Discover the new features and security recommendations in the release announcement. (Skip this info by defining SECURITY_SUGGESTIONS: false)

See detailed reports in MegaLinter artifacts

Your project could benefit from a custom flavor, which would allow you to run only the linters you need, and thus improve runtime performances. (Skip this info by defining FLAVOR_SUGGESTIONS: false)

• Documentation: Custom Flavors
• Command: npx mega-linter-runner@9.6.0 --custom-flavor-setup --custom-flavor-linters ACTION_ACTIONLINT,JSON_JSONLINT,JSON_V8R,JSON_PRETTIER,MARKDOWN_MARKDOWNLINT,MARKDOWN_MARKDOWN_TABLE_FORMATTER,REPOSITORY_CHECKOV,REPOSITORY_GIT_DIFF,REPOSITORY_BETTERLEAKS,REPOSITORY_SECRETLINT,REPOSITORY_TRUFFLEHOG,SPELL_LYCHEE,SPELL_CODESPELL,YAML_PRETTIER,YAML_YAMLLINT,YAML_V8R

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[jklare]

LGTM, but we would need to make sure at least all OSISM employees agree on this and follow those guidelines in their commits. If not, we would need to rephrase "We follow" to "We try to follow" here.

[jklare]

LGTM