Repo clone: multi-repo support

[gatof81]

Follow-up to #188.

Deferred from v1 to keep the initial clone feature lean. v1 ships a single repo per session. This issue adds:

• Schema: replace the singular repo object with repos: [{ … }].
• Validation: only single-repo configs may use target: "" (replace mode); 2+ repos all subpath.
• UI: multi-row layout in the Repo tab with + add repo.
• Tests for the asymmetric subpath cases.

Acceptance

• A session declaring 2+ repos clones each into its declared subpath under the workspace.
• Replace mode is rejected when 2+ repos are declared.
• Existing single-repo configs migrate transparently into a one-element array.