feat(cosh): add skill-dirs configuration field in ~/.copilot/settings.json for custom skill directories

[casparant]

Component

copilot-shell (cosh)

Problem Statement

Currently, cosh loads skills from a fixed set of directories. There is no user-configurable way to specify custom skill directories or control the loading order of multiple skill sources. This limits flexibility for users who want to:

• Add personal or team-specific skill directories
• Override system-level skills with custom implementations
• Manage skills across different environments

Proposed Solution

Add a new configuration field skill-dirs to ~/.copilot/settings.json that allows users to specify an ordered list of skill directories. For example:

{
  "skill-dirs": [
    "$HOME/.copilot/skills",
    "/usr/share/anolisa/skills/"
  ]
}

Key behaviors:

1. Ordered loading: Directories are processed in the order they appear in the array. Earlier paths take precedence over later ones.
2. Path expansion: Support environment variable expansion (e.g., $HOME, ${USER}) and tilde expansion (~).
3. Default fallback: If skill-dirs is not specified, use the current default behavior (system-level directories only).
4. Validation: Warn or error if a specified directory does not exist or is not readable.

Alternatives Considered

• Hardcoding additional default paths — less flexible and harder to customize per-user.
• Using environment variables (e.g., COSKILL_DIRS) — less discoverable and harder to manage alongside other cosh settings.

Additional Context

This change would enable better skill management workflows, especially for:

• Developers maintaining personal skill collections
• Teams sharing common skill libraries
• Users who want to test skill modifications before contributing upstream

The configuration should be documented in the cosh settings reference and included in the default settings.json template (if one exists).