docs(api): drop @brief where it is the only description

[andiwand]

🤖 Generated with Claude Code

Stacked on #840 — the tip of the v7 series, so it applies cleanly. Docs
only; no signature moves.

The argument, and the numbers behind it

@brief marks a short description off from a detailed one. Where there is no
detailed one, it marks nothing — it is ceremony in front of the only sentence
there is. Counting the public headers:

@brief with nothing under it	186
@brief with a real detail paragraph	19
blocks with no @brief at all	65

Two things that count settles. The tag was never applied consistently — a
quarter of the blocks already went without it — and there is no Doxyfile
anywhere in the repo, so nothing is consuming the brief/detail distinction
today anyway.

-/// @brief File not found exception
+/// File not found exception
 struct FileNotFound final : Exception {

The 19 that earn it keep it, and they all look like this:

/// @brief How to decode a file.
///
/// Every field is optional; the default detects everything. @ref as_file_type
/// skips detection outright, @ref file_type_priority only reorders what
/// detection found.

AGENTS.md gets the rule so it stays settled: write the description plainly;
add @brief at the moment a second paragraph arrives.

Wrapping

Stripping seven characters off a first line leaves a multi-line paragraph
wrapped narrower than it needs to be, and ReflowComments does not
re-justify — it only breaks lines that overflow. So the ten affected paragraphs
are rewrapped by hand. Measured both ways: 32 paragraphs could pull a word up
before this change and 32 after, so it introduces no ragged wrapping.

Something the tag was hiding

With @brief gone, filesystem.hpp reads:

/// FileWalker class
class FileWalker {

14 comments are pure tautology — the declaration's own name restated:
/// Represents a document on class Document, /// Represents a text file
on class TextFile, and so on. AGENTS.md already says "Don't restate the
code", so they arguably all want deleting.

I have not touched them here — that is a content change, not this one, and
deleting fourteen doc comments deserves its own review. They read no worse than
they did with the tag on. Say the word and it is a two-minute follow-up.

Verified

Full build clean, no new warnings; 113 targeted gtests pass. Nothing outside
comments changed — the diff is 186 tag removals plus the rewraps.