test: add repl and demo runner 0.5 regression coverage

[rlippmann]

What changed

• Added targeted REPL user-facing regression tests for 0.5 semantics (interactive help/output, confirm vs error clarify rendering, confirmation token variants, idempotent admin output, passthrough/blank/EOF behavior).
• Added demo runner regression checks for renamed 0.5 demo filename mapping and dispatch.
• Added state-validation tests for non-object payload and non-string policy-key rejection paths.
• Marked REPL main() wrapper as no-cover and added focused REPL coverage tests.

Why this was needed

• Recent 0.5 updates changed user-facing REPL and demo runner behavior; these tests lock in expected behavior and reduce regression risk before README work.

Validation

• uv run pre-commit run --all-files
• uv run pytest -q