fix(cli): emit compact tune JSON

[slegarraga]

Closes #431

What

• Adds separators=(",", ":") to tune --json so it matches the compact JSON contract used by other CLI commands.
• Adds a focused regression test that parses the output, checks proposals, and asserts neither ", " nor ": " appears.

Verification

• .venv/bin/pytest tests/unit/test_cli_tune_json.py tests/unit/test_friction_tune.py — 24 passed.
• .venv/bin/ruff check src tests — passed.
• .venv/bin/ruff format --check src/doberman/cli/main.py tests/unit/test_cli_tune_json.py — passed.
• .venv/bin/pytest tests/unit -k 'cli or friction_tune' — 258 passed, 2 skipped.

[slegarraga]

Quick validation status:

• Focused tests, full CLI/friction subset, Ruff check, and format check are green.
• The full parallel suite also reached 91% coverage locally with 3,159 passed; its sole failure was the unrelated plugin-discovery integration case failing to import doberman from its temporary environment.
• Remote CI: package smoke plus Ubuntu Python 3.11/3.12/3.13 are green; Windows is still running. secret-scan fails before scanning because the org-level GITLEAKS_LICENSE secret is missing, matching the pre-existing repository configuration issue.