fix(desktop): clarify add agents channel action

[matt2e]

Summary

• Clarify that the empty-channel intro action adds existing agents to the channel.
• Assert the exact action title and description in the existing E2E coverage while preserving the separate Welcome create-agent flow.

Related issue

None found.

Testing

• env -u BUZZ_AGENT_PROVIDER PATH="$PWD/bin:$PATH" ./bin/just ci
• ../node_modules/.bin/biome check src/features/channels/ui/useChannelIntro.tsx tests/e2e/channels.spec.ts
• ./node_modules/.bin/tsc && ./node_modules/.bin/vite build --mode e2e
• Isolated Playwright case not executed: the sandbox denied the configured local Python web server from binding its port.
• Screenshots not captured; this is a copy-only UI change.