Prefix skill names with plugin name to avoid collisions

[sentry-junior]

Skill names are currently unqualified strings (e.g. github, devinfra, notion). If two plugins or sources define a skill with the same name, the registry will silently collide — last write wins or first registration wins depending on resolution order.

• Skill names should be prefixed with their plugin/provider name (e.g. github/issues, devinfra/docs) to guarantee uniqueness across plugins
• This is a straightforward fix but affects the skill loading path, loadSkill dispatch, and any references to skill names in config or prompts
• No known current collision, but the surface will grow as more skills/plugins are added

Action taken on behalf of David Cramer.