fix: restrict bool env vars to 1/0/true/false

[tmck-code]

Context

_parse_bool previously treated any non-empty env string as True, meaning YAS_FULL_WIDTH=0 and YAS_FULL_WIDTH=false both resolved to True. This made it impossible to explicitly disable boolean flags via environment variables when a yas.toml default had them enabled.

Changes

• _parse_bool in claude/yas/config.py now requires env/CLI boolean strings to be one of 1, 0, true, false (case-insensitive via json.loads). Any other value is rejected and falls through to the config default.
• Added BOOL_ALLOWLIST constant documenting the accepted set.
• Swapped import json in to leverage json.loads for "true"/"false" → bool parsing.
• Tests updated: renamed the old test_env_full_width_any_nonempty_is_true and split into truthy values, falsy values, invalid-falls-through-to-default, env-overrides-toml-true, and max-width-respected-when-full-width-disabled cases.
• Version bumped to 0.2.11.

Checklist

• Tests added or updated for new/changed behaviour
• (bug fixes) a regression test now covers the situation so it can't come back

Screenshots / recording

• N/A — no visible change

Benchmark

• N/A — no performance-relevant change

System info

Key	Value
OS	Linux archlinux 7.0.10-arch1-1 #1 SMP PREEMPT_DYNAMIC Sat, 23 May 2026 14:21:20 +0000 x86_64 GNU/Linux
Claude Code	2.1.167 (Claude Code)
Terminal	TERM=tmux-256color TERM_PROGRAM=tmux SHELL=/bin/bash COLORTERM=truecolor
Locale	LANG=en_AU.UTF-8 LC_ALL=
Python	Python 3.13.1
uv	uv 0.11.14 (x86_64-unknown-linux-gnu)