Make imgen skill sync runtime-aware

[walker1211]

Summary

• Move the canonical imgen skill source to .agents/skills/imgen and keep the repository OpenClaw mirror synchronized.
• Always sync the personal Agents install while skipping optional Claude and OpenClaw installs when their runtime directories are absent.
• Add compatible directory flags, improve root discovery and doctor checks, streamline skill guidance, and clarify per-invocation reasoning effort behavior.
• Package both canonical and OpenClaw skill layouts and add release notes for v0.1.15.

Test plan

• scripts/ci-local.sh clean
• ./skill-sync --check
• Skill structure, eval JSON, release archive, and workflow YAML validation