Move skill-command deduplication by skillName from the Discord-only
`dedupeSkillCommandsForDiscord` into `listSkillCommandsForAgents` so
every interface (TUI, Slack, text) consistently sees a clean command
list without platform-specific workarounds.
When multiple agents share a skill with the same name the old code
emitted `github` + `github_2` and relied on Discord to collapse them.
Now `listSkillCommandsForAgents` returns only the first registration
per skillName, and the Discord-specific wrapper is removed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>