Add simplified-technical-english skill (prose-only ASD-STE100 register)

Desired Behavior

Add a simplified-technical-english skill to the registry. It should guide the agent to write prose in the ASD-STE100 Simplified Technical English register: short active sentences, one idea per sentence, plain and consistent words. It should target readers who need text that is easy to read the first time, such as non-native English readers and ops, deploy, or security instructions where a misread is costly.

The skill must draw a hard line at prose. It must never rename or simplify identifiers, CLI flags, API fields, error strings, commit messages, code comments, or reference documentation.

The registry already has technical-writing (conversational engineering prose) and plain-english-content (GOV.UK content design). This skill adds the STE register and the explicit "prose only, not code" guardrail.

Acceptance Criteria

• skills/simplified-technical-english/SKILL.md exists with a description, license, and keyword triggers (simplified technical english, STE, ASD-STE100).
• The skill states the prose-only boundary and lists what it must not change (code, identifiers, flags, API fields, commit messages, reference docs).
• The skill has a README.md, a .plugin/plugin.json manifest, and the .claude-plugin and .codex-plugin vendor symlinks, matching sibling skills.
• The skill is registered in marketplaces/openhands-extensions.json under the writing category.
• skills/index.js and the README catalog are regenerated, and uv run python scripts/sync_extensions.py --check plus the catalog and manifest tests pass.
• Skill text uses plain hyphens, not em dashes, per the repository convention.

Additional Context

The skill adapts the principles of ASD-STE100 for agent prose. It does not reproduce the standard's controlled dictionary.

Read the existing technical-writing and plain-english-content skills, then inspect the sibling skill layout, marketplaces/openhands-extensions.json, skills/index.js, and the README catalog. Run uv run python scripts/sync_extensions.py --check and the catalog and manifest tests after adding the files, symlinks, registry entry, and regenerated catalogs; done means all acceptance criteria pass.