[Backport 13.4] [TASK] Add AGENTS.md with repository-specific agent instructions

AGENTS.md

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+# AGENTS.md — Frontend Localization Guide
+
+## Repo structure
+
+```
+Documentation/                   # the actual manual (reST source, published to docs.typo3.org)
+CONTRIBUTING.md                  # how to contribute
+```
+
+## Commands
+
+- `make docs` — render the manual locally with Docker
+- `make test-docs` — render in fail-on-log mode; use this to validate any change before committing
+- `pre-commit run --all-files` — apply the whitespace hooks
+  (`trailing-whitespace`, `end-of-file-fixer`) configured in
+  `.pre-commit-config.yaml`; `pre-commit install` wires them into `git commit`
+
+## Scope
+
+This manual covers making a TYPO3 *website* multilingual: site languages,
+page and content translation, language menus, localized labels and character
+sets. Its audience is integrators building a translated frontend.
+
+Do not confuse it with the
+[Localization Team Guidelines](https://github.com/TYPO3-Documentation/localization-team-guidelines),
+which is about the process of translating TYPO3 itself on Crowdin.
+
+Language handling changed substantially across TYPO3 versions, so verify
+site configuration keys, TypoScript properties and fallback behaviour
+against the Core source of the version being documented rather than from
+memory.
+
+## Documentation writing rules
+
+Follow the official TYPO3 documentation writing conventions (see
+https://github.com/TYPO3-Documentation/TYPO3CMS-Guide-HowToDocument):
+
+1. **reST, not Markdown** — everything under `Documentation/` is reStructuredText.
+2. **Sentence case headlines** — first word and proper nouns only:
+   https://docs.typo3.org/permalink/h2document:content-styleguide-title-capitalization
+3. **4-space indentation** for directive bodies, 2 spaces after `..` markers:
+   https://docs.typo3.org/permalink/h2document:cgl-indenting
+4. **Single backticks over double**, unless the content needs a literal
+   backtick: https://docs.typo3.org/permalink/h2document:inline-code
+5. **Every headline needs a `..  _anchor:` target** directly above it
+   (https://docs.typo3.org/permalink/h2document:link-anchor), and anchors are
+   never removed once published
+   (https://docs.typo3.org/permalink/h2document:anchor-persistence).
+6. **Validate before committing** — run `make test-docs`, and run the
+   pre-commit hooks (see Commands).
+7. **Never commit or push without being asked.**
+
+## Commit message format
+
+Follow https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/Howto/EditLocal.html:
+
+- Prefix the subject line with `[TASK]`, `[BUGFIX]`, or `[FEATURE]`,
+  followed by a short, imperative summary.
+- Explain *why* the change is needed in the body — the diff already shows
+  what changed.
+- End with a `Signed-off-by: Your Name <email>` trailer.
+- If AI assistance went beyond basic spelling/grammar checks, add an
+  `Assisted-by: <tool/model name> <contact>` trailer, e.g.
+  `Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>`.
+- If the change should be backported, add a `Releases: main, 14.3, 13.4`
+  trailer listing every branch it applies to. This repo maintains multiple
+  LTS branches, so `Releases:` applies here — including for changes to
+  these agent instruction files themselves, since agents may be working
+  on an older branch where a change is still relevant.
+
+## Pull requests
+
+- When a commit is the only commit in the PR, the PR title and body must
+  match the commit's subject and body exactly.
+- When the commit message has a `Releases:` trailer naming branches beyond
+  `main`, attach the matching `backport <version>` label (e.g.
+  `backport 14.3`, `backport 13.4`) to the PR for each of those branches
+  when opening it — don't wait to be asked.
+- Adding labels requires triage/write access, which an external
+  contributor's account (e.g. a fork-based PR) usually doesn't have. If
+  attaching a label fails for that reason, don't treat it as an error and
+  don't note the failure in the PR — just skip it silently.
+
+## For maintainers
+
+- A PR opened by an agent may be missing its `backport <version>` labels
+  if the agent's account lacked permission to add them. Check for and add
+  any missing backport labels yourself before/when merging such a PR.
+
+## References
+
+- [TYPO3CMS-Guide-HowToDocument](https://github.com/TYPO3-Documentation/TYPO3CMS-Guide-HowToDocument) — official writing style guide and reST reference
+- https://docs.typo3.org/m/typo3/docs-how-to-document/main/en-us/Howto/EditLocal.html — commit/PR conventions

CLAUDE.md

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+@AGENTS.md