Add repository Copilot CLI skill for direct vidpipe command workflows

[Copilot]

The current Copilot extension-wrapper approach for vidpipe tools is brittle (execFile + npx + shell escaping), causing opaque failures and ongoing maintenance overhead. This PR replaces that path with a discoverable Copilot skill that drives vidpipe directly through shell commands and provides usage guidance.

• Skill introduction (.copilot/skills/vidpipe.md)

  • Adds a repository-scoped Copilot skill file that documents direct CLI usage via npx vidpipe ....
  • Includes migration guidance to remove the legacy local extension path (~/.copilot/extensions/vidpipe).

• Complete command/flag reference for targeted workflows

  • Covers all requested subcommands and key flags:
    • ideate --add --topic ... [--no-ai] [--prompt ...]
    • ideate --topics ... [--count N] [--prompt ...]
    • ideate --list [--status draft|ready|recorded|published]
    • schedule, realign [--platform X] [--dry-run], reschedule [--dry-run], review, doctor

• Decision guidance + operational examples

  • Encodes when to use deterministic vs AI-assisted ideation, when to use single-add vs batch generation, and when to preview scheduling changes with --dry-run.
  • Provides copy/paste workflow examples for common operator paths.

# deterministic single idea (no AI enrichment)
npx vidpipe ideate --add --topic "GitOps guardrails for multi-agent repos" --no-ai

# AI-assisted single idea with extra direction
npx vidpipe ideate --add --topic "What this article means for platform engineering teams" \
  --prompt "Reference the article's key finding and propose a contrarian angle."