Support manual-only skill config

[cbusillo]

Summary

• parse upstream-compatible [skills] / [[skills.config]] settings with name and path selectors
• let enabled = false disable implicit skill prompt/routing while keeping skills discoverable and explicitly invokable
• support skills.include_instructions = false to omit the default skills prompt block globally
• document the config shape

Validation

• cargo test -p code-core skills_config_parses_upstream_compatible_selectors -- --nocapture
• cargo test -p code-core config_name_selector_disables_implicit_invocation -- --nocapture
• cargo test -p code-core config_path_selector_disables_implicit_invocation -- --nocapture
• cargo test -p code-core skill_config -- --nocapture
• cargo test -p code-core include_instructions_false -- --nocapture
• cargo test -p code-core selected_skill_messages_include_explicit_dollar_skill_once -- --nocapture
• git diff --check
• ./build-fast.sh

Refs #91