Add Black Candy AI agent integration

[daveliu]

Summary

• add a validated Black Candy agent skill covering setup, discovery, playback, queue, favorites, and playlist workflows
• embed the skill in the CLI and expose blackcandy skill plus blackcandy skill install
• document agent integration and shared skill discovery

Closes #6.

Impact

AI agents can inspect the CLI workflow directly from the binary or install the skill to ~/.agents/skills/blackcandy/SKILL.md. Existing authenticated commands are unchanged.

Validation

• cargo fmt -- --check
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• skill package validation with quick_validate.py
• Cargo package content check
• end-to-end print and isolated install smoke tests

[daveliu]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved. The only conflict was in the unreachable! arm of run_authenticated in src/main.rs — merged Command::Logout (from main's logout command) with Command::Skill(_) (from this branch) into a single arm. Committed as merge commit 9569360.