Support Browser Skill across multiple agents

[mayuqin746]

Summary

• resolve Skill scripts and binaries from the loaded SKILL.md directory while preserving CODEBUDDY_SKILL_DIR as a WorkBuddy compatibility alias
• add browser-cli auth login --client-name with default value Agent and pass it to the authorization URL
• replace the WorkBuddy-specific callback page and credential source metadata with generic Browser CLI wording
• document the cross-agent setup flow and add focused unit tests

Validation

• cargo fmt --all -- --check
• cargo test --all-targets --locked (14 passed)
• cargo clippy --all-targets --locked -- -D warnings
• cargo build --release --locked
• release and Skill shell syntax checks
• Skill ZIP packaging and archive integrity/layout checks

This PR does not bump the release version, create a tag, or publish binaries.