feat: add flare-edge-cli agent skill

[paoloanzn]

Summary

• add a portable flare-edge-cli agent skill under skills/flare-edge-cli
• include focused references for common workflows, AI Worker usage, and install/release behavior
• add agents/openai.yaml metadata for skill UIs
• add a one-line Claude Code skill installer script and document it in the README
• tighten .gitignore so the root binary is still ignored without accidentally ignoring the skill folder

Details

The new skill follows the current Anthropic-style folder format:

• SKILL.md for trigger metadata and core instructions
• references/ for progressive disclosure
• agents/openai.yaml for UI-facing metadata

The installer script copies the skill into ~/.claude/skills/flare-edge-cli by default and supports SKILLS_DIR overrides. The README now includes a copy-paste install command for that path.

Testing

• sh -n install-skill.sh
• local install verification with SOURCE_DIR=/Users/paolo/Repos/flare-edge-cli SKILLS_DIR=<tempdir> ./install-skill.sh

Notes

The README one-liner points at main, so the remote install command will work after this PR is merged.