Handle scp-style SSH remotes correctly in git-mem source add

[Copilot]

Merge-readiness review surfaced a functional blocker: source add misclassified common scp-style Git remotes (git@host:org/repo.git) as local paths and failed with Not a git repository. This PR updates source classification so valid SSH remotes follow the clone path while preserving local-repo behavior.

• Source classification fix

  • cmd_source add now resolves and checks local git repos first (<path>/.git), then treats URL-like inputs (including scp-style user@host:path) as clone targets.
  • Prevents false local-path failures for standard SSH remote syntax.

• Regression coverage

  • Extended tests/test-source.sh with scp-style SSH cases to assert:
    • remote path is handled via clone flow (Clone failed on unreachable host),
    • local validation error (Not a git repository) is not emitted for that path.
  • Uses distinct source names/fixtures to avoid test coupling.

# Before: treated as local path, failed validation
git-mem source add team git@github.com:org/memory.git
# error: Not a git repository: git@github.com:org/memory.git

# After: treated as remote, enters clone path
git-mem source add team git@github.com:org/memory.git
# Cloning git@github.com:org/memory.git...

Original prompt

Review current branch and rate if ready for merge