feat(skills): add social media content calendar

[benbarclay]

Summary

Adds a concrete multi-platform content-calendar task. It turns campaign constraints and verified source material into channel-specific post briefs, copy, asset requirements, editorial status, approval batches, and provider-verified scheduling handoff.

Why this skill

A privacy-preserving Hermes Cloud fleet audit found strong repeated demand for task-level and app-level custom skills. This PR intentionally adds one bounded user task rather than a vague generic workflow or a new model tool. Provider-specific commands remain in existing connector skills.

Validation

• Frontmatter, path/name, size, required sections, and credential-literal checks passed
• tests/tools/test_skill_manager_tool.py: 45 passed
• One new SKILL.md; no core or tool-schema changes

[github-actions]

૮ >ﻌ< ა ci review

_{ran on 353bb97}

all good!

[teknium1]

Merged via PR #81942 with your commit cherry-picked onto current main — authorship preserved in git log. Shipped to optional-skills/creative/ (vertical audience) with polish per the skill authoring standards; the publish step now distinguishes provider-verified publication from scheduler handoff. Thanks!