docs(feat[requirements]): document sphinx 8.1+ floor and typed domain idiom

why: Record the new Sphinx 8.1+ workspace floor where contributors
and downstream users look for it, and codify the typed-accessor
convention so the `env.get_domain(<literal>)` / `t.cast` pattern does
not creep back in. CLAUDE.md is a symlink to AGENTS.md so both
surfaces update in one edit.
what:
- AGENTS.md: add `Sphinx 8.1+` to the Development Environment list;
  add a new `Sphinx domain access` Coding-Standards subsection
  explaining `env.domains.<name>_domain` and listing the core
  typed accessors
- README.md: add a top-level `Requirements` section (Python 3.10+,
  Sphinx 8.1+) above the install instructions
- packages/sphinx-autodoc-fastmcp/README.md: tighten the
  `Dependencies` list entry from `Sphinx` to `Sphinx 8.1+`

Author: tony

AGENTS.md

@@ -26,6 +26,7 @@ Key features:
 
 This project uses:
 - Python 3.10+
+- Sphinx 8.1+ (required for the typed `env.domains.<name>_domain` accessors)
 - [uv](https://github.com/astral-sh/uv) for dependency management
 - [ruff](https://github.com/astral-sh/ruff) for linting and formatting
 - [mypy](https://github.com/python/mypy) for type checking
@@ -448,6 +449,22 @@ Key highlights:
 - **For typing**, use `import typing as t` and access via namespace: `t.NamedTuple`, etc.
 - **Use `from __future__ import annotations`** at the top of all Python files
 
+### Sphinx domain access
+
+Prefer the typed accessors on `env.domains` over `env.get_domain(<literal>)`:
+
+- `env.domains.standard_domain` — not `env.get_domain("std")`
+- `env.domains.python_domain` — not `env.get_domain("py")`
+- Similarly: `c_domain`, `cpp_domain`, `javascript_domain`,
+  `restructuredtext_domain`, `changeset_domain`, `citation_domain`,
+  `index_domain`, `math_domain`
+
+The typed accessors return the concrete domain subclass
+(`StandardDomain`, `PythonDomain`, etc.), so mypy sees subclass-specific
+attributes (`progoptions`, `add_program_option`, `data["objects"]`, …)
+without `t.cast` or `# type: ignore`. The accessors were added in Sphinx
+8.1 (`_DomainsContainer`), which is the workspace floor.
+
 ### Docstrings
 
 Follow NumPy docstring style for all functions and methods:

README.md

@@ -4,6 +4,11 @@ Integrated autodoc design system for Sphinx.  Twelve packages in three tiers
 that replace ~300 lines of duplicated `docs/conf.py` with ~10 lines and
 produce beautiful, consistent API documentation.
 
+## Requirements
+
+- Python 3.10+
+- Sphinx 8.1+
+
 ## Install
 
 ```console

packages/sphinx-autodoc-fastmcp/README.md

@@ -43,7 +43,7 @@ See the package docstrings and `sphinx_autodoc_fastmcp.setup()` for defaults.
 ## Dependencies
 
 - Python 3.10+
-- Sphinx
+- Sphinx 8.1+
 - `sphinx-ux-badges`, `sphinx-ux-autodoc-layout`, and `sphinx-autodoc-typehints-gp`
   are declared dependencies and installed automatically with this package.