feat: add Skill Hub for per-agent skill management

Introduces a skill marketplace that lets users toggle workspace
capabilities (files, browser, todos, timers, routines, tunnel) per
agent — from the workspace UI, the launcher CLI, or daemon config.

Backend: skill catalog module, enabled_skills JSONB on WorkspaceMember,
GET /v1/skill-catalog endpoint, PATCH member accepts enabled_skills.

Launcher: JS skill catalog, `agn skills` CLI command, daemon reads
skills config instead of hardcoded empty Set, MCP server and prompt
builder now gate todos/timers/routines on disabledModules.

Frontend: SkillCatalogEntry type, API methods, AgentSkillsSection
component with toggle switches integrated into agent profile panel.

Adapter syncs workspace-managed skills on connect so workspace
settings are authoritative over local config.