docs: bound documentation and comment length in template AGENTS.md (port aeloop#434)

[mariushelf]

Port the added-fact documentation rule from
XomniaAI/agentic-engineering-loop#434
into the template's AGENTS.md, so generated projects tell their agents to keep
docs, docstrings, and comments short.

Why

The template already enforces docstring presence — ruff D with the numpy
convention in template/pyproject.toml.jinja — but nothing bounds their
length. Agents working in a freshly generated project therefore produce
Parameters/Returns blocks that restate the annotations, step-by-step
narrations of the implementation, and docstrings that only grow on every edit.
Upstream #434 fixed exactly this gap in the loop's own AGENTS.md with a single
rule: past the summary line, a line must carry a fact the code cannot.

template/AGENTS.md is the natural home — it is the file template/CLAUDE.md
includes (@AGENTS.md), and it currently holds only Package Management and
Git Conventions.

Scope

Two files, both under template/:

1. template/AGENTS.md — append the new section (see verbatim text below).

2. tests/test_template.py — in test_template_renders, add coverage that the
   agent-instruction files actually land in a generated project:

   assert (project_path / "AGENTS.md").is_file()
   assert (project_path / "CLAUDE.md").is_file()

No other file changes.

Exact text to add

Append at the end of template/AGENTS.md, after the Git Conventions
section, separated by one blank line. Heading is Title Case to match the two
existing headings (upstream uses sentence case; the template does not). Prose
wrapped at 80 columns. template/AGENTS.md is a static file (no .jinja
suffix), so it must contain no Jinja expressions — do not reference
{{ project_slug }} or any other variable.

## Documentation and Comment Style

Brevity is not tidiness — it is what gets the text read. A docstring, comment,
or page nobody reads still has to be maintained, so it is worse than none.

Docstring *presence* is enforced by ruff's `D` rules (numpy convention) in
`pyproject.toml`. This section bounds their *length*.

The unit of judgement is the *added fact*: after the summary line, every line
must tell the reader something they cannot get from the name, the signature,
the type hints, or the code itself.

Earns more than a summary line:

- a contract the signature doesn't show — preconditions, invariants, what it
  raises
- units, bounds, or formats a type can't carry (`timeout: float` — seconds or
  milliseconds?)
- caller-visible behaviour that would surprise — mutation, ordering,
  idempotency, blocking, retries, cost (a network round-trip, an LLM call)
- a pointer to the *why* when it isn't obvious (link the issue or ADR, don't
  restate it)

Does not:

- restating the signature, or a numpy `Parameters`/`Returns` section where the
  name and type already say it
- narrating the implementation step by step — that's the code
- history ("previously…", "refactored to…") — that's git
- examples for a function whose use is obvious from its name

The same rule governs prose: the README covers what a reader needs to install,
run, and maintain the project; implementation detail belongs in the code.
Prefer a list, table, or short code example over paragraphs. When editing an
existing docstring, comment, or page, it must not get longer unless behaviour
was added.

Deliberately not ported from #434

• The docs/source/contributing/code_style.md half. The template ships no
  Sphinx docs/ tree and no contributing guide, so that counterpart has no
  target here. AGENTS.md is the single source.
• Loop-specific wording. "the cost of spawning a Claude run" becomes the
  generic "a network round-trip, an LLM call"; "docs/ covers behaviour needed
  to operate or maintain the loop" becomes the README sentence above.
• The port-inheritance rewording — specific to the upstream codebase (also
  dropped by #434 itself relative to its own upstream).

Out of scope

• Adding a root-level AGENTS.md for the copier_python repo itself.
• Adding a docs/ tree or CONTRIBUTING.md to the template (that belongs in
  its own issue if wanted).
• Changing the ruff D selection or its ignore list in
  template/pyproject.toml.jinja.
• Retro-fitting the rule onto the template's existing docstrings in
  template/src/{{ project_slug }}/*.jinja — they are already one-liners and
  comply.

Acceptance criteria

• template/AGENTS.md ends with the Documentation and Comment Style
  section, matching the text above; the existing Package Management and
  Git Conventions sections are unchanged.
• No Jinja delimiters ({{, {%) introduced in template/AGENTS.md.
• tests/test_template.py::test_template_renders asserts both AGENTS.md
  and CLAUDE.md exist in the generated project.
• uv run pytest tests passes (all tests, including the rendered
  project's make lint, make test, and pre-commit run).
• Commit message uses the docs: prefix and carries no Co-Authored-By or
  other AI attribution trailer, per the template's own Git Conventions.

Verification

cd /workspace/copier_python
uv run pytest tests -q

The suite generates a project into a temp dir, runs uv sync, then make lint,
make test, and pre-commit run --all-files inside it — so it needs network
access and takes a few minutes. Nothing in the added text is executable, so a
green suite plus a read of the rendered AGENTS.md is the full check.

Risk

Low. Documentation-only text in a static template file, plus two assertions.
Worst case is wording taste, reviewable in the diff.

[mariushelf]

~Written by Claude, run via the agentic engineering loop

[mariushelf]

Branch feat/9-loop-docs-bound-documentation-and-comment-len pushed; opening the PR next.

~Written by Claude, run via the agentic engineering loop