bug(general-doc-constraints): missing Unicode box drawing exception for tree output

[KemingHe]

What Happened?

Expected

GDC Characters constraint should explicitly permit Unicode box drawing characters used in tree output since directory structure rendering via tree --dirsfirst is a documented pattern in doc-generation skills.

Actual

Characters constraint says "QWERTY keyboard typeable only" with only one named exception (arrow for ToC navigation). Box drawing characters used in tree output are technically non-QWERTY, creating ambiguity when generating directory structures.

Steps to Reproduce

1. Use a doc-generation skill that renders directory structure.
2. Skill follows tree --dirsfirst convention per its own instructions.
3. Output contains box drawing characters that violate the Characters constraint.
4. Agent must make ad-hoc exception decisions per invocation.

Proposed Fix

• Add box drawing characters as a named exception in the Characters constraint, alongside the existing arrow exception.
• Bump GDC to v1.2.0.