fix(cli): pass --model through to the TUI entry

[holny]

Issue for this PR

Fixes #47172

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

opencode --model <id> --prompt ... accepted the flag on the root command but dropped it before the TUI started, so the session silently ran on the configured default model — the requested id was never mentioned or validated. The run path already resolves and validates the same flag.

• Define --model (with -m alias) on the root command params
• Pass it through to the TUI args, where the existing args.model handling parses it and selects the model, warning on malformed values — matching the run path

How did you verify your code works?

• --help on the root entry now lists --model, -m
• bun typecheck in packages/cli and packages/tui
• bun test in packages/cli (259 passing) and packages/tui (2 pre-existing timeouts in dialog-shell-output tests reproduce on a clean baseline, unrelated)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

---

Supersedes #47699 — the PR head fork was accidentally deleted on 2026-09-09 (not intentional, see the notification comment there); re-filing so the review can continue. Original conversation: #47699

[github-actions]

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