Add --provider and --model CLI flags for one-shot overrides

[Osso]

Summary

Let users pick the provider and/or model for a single launch from the
command line, without editing config or opening the TUI's
`/config` / `/provider` / `/model` flow.

Useful for shell aliases, scripted launches, and ad-hoc model swaps:

```
ask --provider codex --model gpt-5
ask --provider claude
alias askc='ask --provider claude --model claude-sonnet-4-6'
```

Happy to PR; carrying it locally already.

Behavior sketch

• `--provider ` / `--provider=` — set the provider for this run only,
  applied after `saveConfig(cfg)` so it never persists. Unknown ids exit
  with status 2 and an error listing the registered ids; resolve via a strict
  registry lookup so a typo does not silently fall back to the first provider.
• `--model ` / `--model=` — set `first.providerModel` after
  `newTab` returns, so the override beats the provider's saved settings
  without modifying them. No validation: the in-TUI model picker already
  supports custom strings, so anything the picker would accept is legal here.
• Bare `--provider` / `--model` (no value) and empty `--provider=` /
  `--model=` are validation errors so a typo doesn't swallow the next
  positional arg.
• Parse the new flags before the existing `resume` / `--help` / `_hook`
  dispatch so they don't trip an unknown-option check.
• `printHelp` documents both flags under an `Options:` section.

Tests worth adding

• known provider id → applied
• unknown provider id → exit 2 with helpful error
• bare `--provider` (no value) → validation error
• `--model foo` → `first.providerModel == "foo"`
• `--provider claude --model bar` → both applied

Notes

This pairs naturally with #4 (`ask resume ` ignoring `LastProvider`):
once both land, a user can do `ask --provider codex resume ` to
explicitly force a cross-provider resume, and the no-flag case picks up
`LastProvider`.