Doghouse: fail fast on malformed --repo values

[flyingrobots]

Follow-up to #4.

Why

The CLI help advertises --repo owner/name, but resolve_repo_context() currently accepts slashless input and silently rewrites it into owner=name=repo.

Example:

• --repo wesley becomes wesley/wesley

That is not a helpful fallback. It is a hidden query rewrite.

Current behavior

Relevant file:

• src/doghouse/cli/main.py

snapshot, watch, and export all share the same repo-context resolver now, which is good. But malformed input still gets normalized instead of rejected.

Desired outcome

If the user supplies --repo, it should either:

• be a valid owner/name, or
• fail immediately with a clear error message

Auto-detection should remain the fallback only when the user does not provide a repo explicitly.

Acceptance criteria

• Invalid explicit --repo values fail loudly.
• snapshot, watch, and export all share the same validation path.
• Add regression tests for malformed explicit input.
• The CLI help and actual behavior stay aligned.