feat: derive goal hash from goal

[fazpu]

Summary

• remove goal_slug from root config and init/example templates
• derive a deterministic goal_hash from the configured goal
• use goal_hash in session ids, session metadata, loop state, and config snapshots
• update docs and tests for the derived tracking identifier

Verification

• uv run pytest
• uv run ruff check src/loopy_loop src/tests
• uv run ruff format --check src/loopy_loop src/tests