feat: add `skill` CLI command and `.opencode/tools/` auto-discovery

[anandgupta42]

Summary

Add a top-level altimate-code skill CLI command with list, create, and test subcommands, plus auto-discovery of user CLI tools in .opencode/tools/ on the agent's PATH.

Motivation

Enable users to extend altimate-code by pairing SKILL.md files with simple CLI tools (bash/python/node scripts). This is the foundation for a skill + CLI tools extension ecosystem that's simpler and more reliable than MCP servers.

Changes

• skill list — shows all skills with detected paired CLI tools, source, description
• skill create <name> — scaffolds SKILL.md + CLI tool stub (bash/python/node)
• skill test <name> — validates skill frontmatter + checks paired tool on PATH + runs --help
• Auto-prepend .opencode/tools/ (project) and ~/.config/altimate-code/tools/ (global) to PATH in bash tool and PTY sessions
• Updated docs for skills and custom tools