fix: create local config when index dir exists

[srobinson]

Summary

• Route mdm init through local config creation when .mdm/ exists but .mdm.toml is missing.
• Keep the existing no-op behavior when both .mdm/ and .mdm.toml already exist.
• Add regression coverage for the missing-config rough edge and the both-exist no-op case.

Test plan

• Added failing regression before implementation.
• npx --yes pnpm@10.28.0 check
• npx --yes pnpm@10.28.0 build
• npx --yes pnpm@10.28.0 test
• Manual smoke: pre-create .mdm/ without .mdm.toml, run mdm init --local --yes, verify Created .mdm.toml and file exists.