docs(talk-like-scarletkc): loosen tweet voice guidance

[scarletkc]

The current Social guidance can over-polish casual tweets and constrain how facts and update highlights are presented. This update captures recent writing feedback in the existing Social mode.

• Keep natural wording, short phrases, and asides. Let punctuation follow readability and allow casual paragraph endings, while keeping technical explanations clear.
• Allow factual openings and first-person wording where it fits. Select update highlights without a fixed count.
• Preserve the voice and central point of an approved tweet during small edits, and return the integrated draft unless a narrower edit was requested.
• Add two behavioral evaluation cases for casual rhythm and clear technical comparisons.

The instructions remain short and specific to the author's preferences. They do not prescribe punctuation quotas or a fixed tweet template.

Validation:

• python -m unittest discover -s tests -v: 77 tests passed.
• python scripts/update_readme_skills.py --check: passed.
• Skill structure validation and git diff --check: passed.
• Independently generated and reviewed both new writing cases for natural tone and preservation of technical conditions. These are qualitative checks, not a measured improvement score.
• gh skill publish --dry-run could not run locally because the GitHub CLI is unavailable; the repository workflow runs this check.