Example style guide, 80-column examples, tested doc fences

[tony]

• StyLua formats examples/ at 80; the rest of the source keeps 100.
• A stdlib Python check holds Markdown fences and comments to 80, which StyLua never reads or wraps.
• A fence harness runs every lua fence in the README and docs (none ran).

Merge after libtmux/docs#101 deploys: it makes 80 columns fit a libtmux.org code block from 1024 px up.

What changed

Policy

• .github/WRITING.md gains an ## Examples section after ## Markdown and code blocks (now with a ### Code blocks heading the shared text links to). The existing example-shape rules stay under ## Documentation and examples. The "In this repository" block names StyLua, stylua.toml, examples/stylua.toml, and the <!-- lua: ... --> fence classes.
• AGENTS.md routes code examples to it.

Tooling

• examples/stylua.toml is a copy of the root config with column_width = 80. StyLua starts in the directory of the file being formatted and searches upward, so the existing stylua --check needs no new flags. A [[formatter_copies]] entry fails the check if the copy differs from the root in anything but column_width.
• scripts/check_example_width.py and .github/example-width.toml read every **/*.md page, examples/**/*.lua and examples/api/run.sh. Two [[exclude]] entries, each with a reason: examples/stylua.toml, which [[formatter_copies]] already compares, and examples/api/manifest.json, a machine-read index whose JSON strings cannot wrap. One [[allow]] entry: the deliberately wide Bad line in WRITING.md's port block. They run as the width gate inside scripts/check.py mid: self-test first, then the check.
• scripts/check_doc_fences.py (the fences gate, outer phase): each lua fence carries <!-- lua: run | fragment | compile-only: reason --> and an unclassified fence fails. The 11 existing fences are 1 run, 7 fragments, 3 compile-only.
• The fences gate runs in the outer phase, not mid, because it needs a tmux 3.7c binary.

Examples

• One mechanical StyLua reformat of examples/**/*.lua (10 Lua files, +124/-45, plus the new examples/stylua.toml); the hand edits name session, query and split options before the call, and split two shell commands.

Evidence

• StyLua 2.4.0 (the version ci.yml installs) takes the width from the nearest config (search order, column_width). column_width guides the formatter and does not cap a line, which is why the separate width check exists.
• Rejected a root column_width of 80, which would reflow lua/ and packages/ for no reader benefit; Examples(style): Format examples at 80 columns weighs the other options.
• StyLua needs two passes on a method chain inside a call argument in examples/quickstart.lua: the second pass rewrites the first pass's output with identical flags and config. Examples(style[quickstart]): Name session options hoists that chain into a local.
• Run against origin/master, this branch's checker and config report 51 lines over 80 in 15 files, and the commits fix all of them.
• Fence harness: each fence is classified and run; matching fences line for line against example programs was rejected because only 1 of 11 matched. A planted pane:capturee in docs/panes.md fails with "attempt to call a nil value (method 'capturee')"; the clean tree passes. The gate needs TMUX_BIN at the 3.7c toolchain the repository bootstraps, because libtmux-lua does not support tmux 3.7d yet.

How to verify

$ mise exec -- python scripts/check_example_width.py --self-test

$ mise exec -- python scripts/check.py width

$ mise exec -- python scripts/check.py format

The fence harness needs the repository's tmux 3.7c in TMUX_BIN.

$ mise exec -- python scripts/check.py fences