feat(skills): support package specifier in generated SKILL.md

mcp add has mcp.command to override the registered command (e.g. bunx @my-scope/my-cli --mcp instead of bunx my-cli --mcp).

skills add has no equivalent. Generated SKILL.md files always use the bare CLI name from Cli.create(), so skill files reference my-cli foo instead of bunx @my-scope/my-cli foo. This only works if the user has the package installed globally.

Proposal: add a command option to the sync config that controls how the CLI name appears in generated skill content.

Cli.create("my-cli", {
  mcp: { command: "bunx @my-scope/my-cli --mcp" },
  sync: { command: "bunx @my-scope/my-cli" },
})

When set, Skill.ts would use this value instead of the bare name when rendering command signatures, examples, and the --llms footer.

Start by tracing the sync configuration into Skill.ts, where generated SKILL.md command signatures, examples, and the --llms footer are rendered. Verify that a configured command replaces the bare CLI name in all three places and that the existing bare-name behavior remains when no command is set.