fix(registry): preserve existing commands on alias conflicts

[hiSandog]

Description

A command alias could silently overwrite an already registered command with the same site/name. That made adapter and plugin load order change which implementation ran for an established command.

The registry now ignores only the conflicting alias while continuing to register any remaining unique aliases. A regression test proves that the original command remains routable and a unique alias still works.

Related issue: None.

Type of Change

• 🐛 Bug fix

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Screenshots / Output

npx vitest run --project unit src/registry.test.ts
22 passed

npm run typecheck
tsc --noEmit

Note: the full npm test run was also attempted. Its unrelated plugin/download/adapter failures require local listener or download capabilities unavailable in this environment; the focused registry suite and typecheck pass.