feat(demo): rich-UX rewrite + asciicast recording + README embed

[kmadan]

Summary

Two commits, one bundled PR. PyPI re-release deferred — this is README-visible polish + the underlying canonical-path improvements that benefit every CLI user.

1. `feat(demo): rewrite for canonical rich-UX flow`

The v0.7.1 demo runner shipped with plain `print()` calls — the banner, spinners, MessageGroup, and success markers that ship in `examples/quickstart.py` were entirely missing. This rewrites the runner around the high-level `application.create()` + `app.evaluate()` API and wires in `print_banner`, `spinner`, `MessageGroup`, `info`, and `success` from `aicertify.utils.logging_config`. The demo now looks identical to what `examples/quickstart.py` produces.

Three supporting fixes keep the rich UX legible:

• CLI default verbosity → WARNING (was INFO). `aicertify demo` and `aicertify evaluate` no longer flood with INFO chatter from langfair / deepeval / OPA policy loader / transformers. `--verbose` opts back in.
• `policy_loader` no longer warns on `helper_functions/` — those `.rego` files are shared library code, not policies, always meant to be skipped silently.
• Stderr capture inside the demo runner — heavy ML evaluators emit ERRORs for missing OPENAI_API_KEY (expected in no-key demo path; OPA verdict is the substance). Runner redirects fd 2 to a tempfile and only replays it on non-zero exit.

2. `docs(demo): add asciicast recording + GIF + README embed`

Real recording of `aicertify demo` running end-to-end:

• `docs/demo.cast` — asciinema v2 cast (60 KB, 135 frames). Replayable byte-exact via `asciinema play`.
• `docs/demo.gif` — agg-rendered GIF (316 KB, 2× speed, monokai). Embedded in README right after the Quick Start so visitors see the rich UX before installing anything.

Recorded against the rich-UX runner from the preceding commit, with `OPENAI_API_KEY` unset — so it shows what a fresh-install user actually experiences.

Test plan

• Local test in fresh venv (`/tmp/aicertify-demo-test`, editable install): `aicertify demo` produces clean rich-UX output, generates valid Markdown report
• Same with `OPENAI_API_KEY` unset → same rich UX, OPA verdict in report (heavy evaluators skip gracefully)
• `aicertify --verbose demo` → restores INFO/DEBUG output (verifies opt-in path)
• `aicertify demo --format json` → still works (JSON path)
• Legacy invocation `aicertify --contract X --policy Y` still routes to `evaluate` (back-compat preserved)
• GIF visually inspected via extracted frames (banner, spinners, MessageGroup, completion markers all captured)
• Both commits clean — zero Co-Authored-By trailers

Not in this PR

• PyPI re-release — deferred. Main has rich UX; PyPI still has v0.7.1 plain UX. Will bundle v0.7.2 with future work (more examples, MCP, etc.) rather than re-release per change.