bug(readme-creation): consolidate fragmented skill into clean v3.2.0

[KemingHe]

Related

• bug(general-doc-constraints): missing Unicode box drawing exception for tree output #99: GDC Unicode box drawing exception

What Happened?

Expected

readme-creation skill should be a self-contained SKILL.md plus a single template. Default output should reflect current best practices without requiring user to reference migration guides or supplementary files.

Actual

• Skill is fragmented across SKILL.md, readme-template.md, and upgrade-from-v3-0-0.md.
• Users must explicitly attach migration guide context to avoid legacy patterns.
• "No Last Updated line" is buried in a migration guide rather than being default template behavior.
• Directory tree ordering convention (dirs first, then files, both alphabetical) is documented in SKILL.md but not in the template itself.

Steps to Reproduce

1. Invoke readme-creation skill without referencing migration guide.
2. Output may contain legacy patterns (Last Updated line, incorrect ordering).

Proposed Fix

• Absorb all migration content into SKILL.md as native instructions.
• Remove separate migration guide files.
• Template defaults to no Last Updated line (not as a rule, just absent from template).
• Add explicit ordering rules to template.
• Target clean v3.2.0 release.