feat: add solver supervisor stop dry run

[SaJaToGu]

Refs #223

Third small PR for the solver process supervisor split.

Scope:

• adds solver_supervisor.py stop --dry-run
• supports selecting exactly one run by run id, repo plus issue, or worker pid
• previews known runner and worker PIDs plus child process tree
• sends no signals and does not stop processes
• rejects non-dry-run stop attempts in this slice

Validation:

• ./.venv/bin/python -m unittest tests.test_solver_supervisor
• ./.venv/bin/python scripts/solver_supervisor.py stop --run-id 20260604-232546-262166-ai-issue-solver-issue-189 --dry-run
• ./.venv/bin/python -m unittest discover -s tests
• git diff --check

Follow-up PRs for #223 should add real safe stop with worktree preservation and dashboard integration separately.