Comment and docstring conventions, and a pass applying them

[pingviini]

Comments and docstrings only. No behaviour change.

The convention, written down

AGENTS.md gains a Comments and docstrings section. The rules are written against this
codebase's habit of explaining at length, because that licence is what makes empty prose expensive:
a docstring opening with four lines that restate the signature teaches a reader to skim the
paragraph above it, which is where the reasons live.

Six rules: write what the code cannot say; say it once, where the decision lives; no :param: or
:return: that restates the signature, but keep every :raises:; cross-file pointers rot; bug
histories belong in the commit message; a docstring's first line says what the thing is for.

Applied to the comments added in #6

The same argument — why both format sets are published, so a mirror never reads "not pdf, therefore
image" — was made at OUTPUT_MODELS, again on ImageOutputRules.formats, again on
PdfOutputRules.formats, and again in the document builder. Now made once, where the sets are
defined.

The comment above reported in app/main.py ran ten lines: the rule, the bug behind it, the
Pydantic validator ordering that makes the bad case unreachable today, and what would happen if a
check moved into a field validator on files. Four lines remain — which error answers for
detail, code and rule, and why it is not simply errors[0]. Also gone: a comment naming
tests/test_output_rules.py from output_rules.py, and two test docstrings that restated the
assertions below them.

Applied to the docstrings

Twenty-four :param: lines existed; most restated the name and the annotation — :param renderer: The Typst renderer instance, :param count: The number of keys the caller sent. Removed where
they restate, kept where they add what the annotation cannot: that key reaches the caller's error
context, that source_bytes is already UTF-8, that _resource_limit_args returns empty when the
limit is unset, that a dsn of None disables Sentry. Every :raises: is kept — exceptions are
not in the signature, so those lines are the only statement of what escapes.

This pass covered app/render/renderer.py, app/api/routes.py and app/render/templates.py, which
held 20 of the 24. app/core/sentry.py and app/core/errors.py keep theirs, all of which carry
information.

Verification

uv run pytest (717 passed, 2 skipped, integration included), uv run ty check app scripts,
uv run ruff check . and make docs-build all pass.