Drop the header from the default instance profile README

The starter profile/README.md that vole admin init writes into the instance repository repeats the landing page heading (# <instance_name>) and says little else, so a new instance shows the forge name twice and no hint about how to replace the text or where the other instance files live.

Scope

• Drop the header from the generated profile/README.md: the home page already renders instance_name as the heading.
• Replace it with one short line that names profile/README.md as the file to edit and links to the instance repository’s own overview page (/r/<repo>).
• Build the link from the configured instance_repo name rather than assuming vole-instance, and keep a plain-text fallback when no name is available.
• Pass the repository name through instancerepo.Ensure/Template and update the two callers; the instance name is no longer needed by the template.

Acceptance criteria

• A freshly scaffolded instance repository has a profile/README.md with no markdown heading.
• The file explains how to customize the introduction and links to the instance repository’s overview at /r/<instance_repo>.
• Existing repositories with history are untouched.
• go test ./forge/instancerepo/ covers the headerless starter and the link.