feat(skills): add general X content writing skill

[scarletkc]

X topic selection and post structure currently live inside a personal voice skill. Add a standalone x-content skill for discovering material, choosing angles, writing posts and threads, and checking the result from the intended reader's perspective.

The guide supports updates, technical explanations, personal experiences, opinions, and replies. It adjusts background to reader familiarity and chooses short posts, longer posts, or threads according to the content. Reader checks cover missing context, useful development, natural endings, and repeated caveats.

Move X-specific guidance and evaluation cases out of talk-like-scarletkc, retaining its voice rules and a single link to the content skill. Voice-only edits and faithful translations continue without restarting topic discovery. Add the generated README catalog entry and behavioral cases for the new skill.

Validation:

• uv run --python 3.12 python -m unittest discover -s tests -v — 77 tests passed.
• uv run --python 3.12 python scripts/update_readme_skills.py --check.
• gh skill publish skills --dry-run.
• Skill Creator quick_validate.py for both changed skills.
• Evaluation JSON, local Markdown links, and git diff --check.
• Independent writing and reader checks covering technical content, unfamiliar and familiar products, personal expression, flexible post length, and use alongside the personal voice skill.