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

AGENTS.md

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+# AGENTS.md — Contribution Guide: Core Development
+
+## 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 documents the *process* of contributing to the TYPO3 Core:
+accounts, Gerrit and the review workflow, coding guidelines, bug fixing,
+testing, and the responsibilities of Core mergers. It is about how people
+work together, not about the TYPO3 API.
+
+Two consequences follow:
+
+- The facts to verify are the **process and the tooling** — Gerrit, Forger,
+  forge.typo3.org, the relevant GitHub repositories — not PHP signatures.
+  These change independently of TYPO3 releases, so check the real service
+  before describing a workflow.
+- **Do not confuse the two contribution workflows.** Changes to the TYPO3
+  Core itself go through Gerrit; changes to the documentation go through
+  GitHub pull requests. This manual describes the former but is itself
+  edited through the latter.
+
+## 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>`.
+- **No `Releases:` trailer.** This manual is unversioned: it has only a
+  `main` branch and no LTS branches, so the backporting conventions and
+  `backport <version>` labels described in the how-to-document guide do
+  not apply here.
+
+## 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.
+
+## 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