Demo polish + download-models.sh helper

[FrancoisChastel]

Summary

Two related changes that make the project more presentable to a first-time user.

1. Demo: drop git, contextual question, three-tier showcase

Problems with the prior demo

• Beat 3 asked about git stash inside a non-git sandbox — the framing felt contrived even when the model answered cleanly.
• Only two tiers ever appeared on screen (fast as the implicit default, one `--balanced` for the piped diagnosis), even though the pitch is "you can pick fast, balanced, or smart per call".

Changes

• Beat 0 is now `?? --status` instead of `?? --start fast`. It prints all three tiers as up with their ports (`fast → :8091`, `balanced → :8080`, `smart → :8093`), so the multi-tier capability is visible from the first frame.
• Beat 3 rewritten to `? in 2 lines, what does the -k flag mean in du` — a flag the audience just saw in Beat 1's `du -k` output. Natural follow-through.
• Beat 4 routes to smart (`? --smart`) instead of balanced. Qwen3-Coder-Next is the right fit for the piped Python traceback, and this completes the fast/balanced/smart triad on screen.
• Archive seed swapped to a Docker volumes conversation, and the recall (`??? docker volume`) finds it. The feature now shows its real value — a relevant past conversation surfacing from days ago — without forcing a git context.

2. `scripts/download-models.sh` — one-command model fetch

The install used to say "huggingface-cli download <long-repo-name>". Easy to typo, hard to discover the other tiers from. The new helper wraps the same call with the tier registry baked in:

```sh
scripts/download-models.sh # default: balanced (~16 GB)
scripts/download-models.sh fast # one specific tier
scripts/download-models.sh fast balanced smart # multiple
scripts/download-models.sh all --with-embed # everything + default embed
scripts/download-models.sh --list # what's downloaded vs not
```

The `--list` view shows every tier (chat + embed), on-disk status (✓/✗), the HF repo path, and which is the default. Same information `?? --list` prints, but available before you've sourced the zsh layer. Re-downloads are skipped (HF cache is content-addressed), so the script is safe to re-run.

• Homebrew formula installs it executable and points caveats at it.
• README install step 2 now calls the helper instead of the raw `huggingface-cli` command.
• CONFIGURATION.md's model-tiers section documents it.

bash 3.2 compatible (macOS default shell).

Test plan

• 260 tests pass (`pytest -q`)
• `ruff check` and `ruff format --check` clean
• Demo renders end-to-end with three tiers visible in `?? --status`, contextual du question, `--smart` switch, Docker recall hit
• `download-models.sh --help` and `--list` render correctly under bash 3.2 (macOS default)
• Manual: `brew install --build-from-source ./Formula/shellllm.rb` and verify `brew test` covers the new helper