fix(cli): stop bare reasonix from re-running the setup wizard

[esengine]

Problem

Running a bare reasonix (no subcommand) could re-trigger the full setup wizard even after the user had already configured — re-asking language / provider / API key, and potentially overwriting their config (#2856).

Two paths caused it:

1. welcome() auto-launched interactiveSetup whenever config.SourcePath() == "". But SourcePath() only checks the cwd reasonix.toml and the global ~/.config/reasonix/config.toml. reasonix setup writes a project-local reasonix.toml, so running reasonix from any other directory makes SourcePath() return "" → the wizard fires again.
2. chatREPL ErrUnknownModel recovery re-ran the wizard whenever the configured default model failed to resolve — even when a config file clearly existed — silently overwriting it.

Fix

• welcome() no longer auto-launches the wizard. With no resolvable config it falls through to the existing status banner, which already lists the next step (reasonix setup). Explicit reasonix setup is unchanged.
• The ErrUnknownModel recovery now only re-runs setup on a true first run (config.SourcePath() == ""). When a config exists but its model can't resolve, it surfaces the existing descriptive error (which already says to run reasonix setup) instead of overwriting the config.

No behaviour change to reasonix setup, reasonix chat, or any non-interactive path.

Verification

• go build ./..., go vet ./internal/cli/..., go test ./internal/cli/... — pass.

Closes #2856

[esengine]

Closing in favour of the first-run rework that already landed on main-v2.

#2856's root cause was that reasonix setup wrote a project-local reasonix.toml, so running reasonix from another directory couldn't see it and re-triggered the wizard. main-v2 now fixes that at the source: setup writes to the user-global config by default, and welcome() only auto-launches the wizard when no config resolves from any source (src == "" && cfgErr != nil). A configured user always loads config and drops straight into chat — no more surprise wizard, no overwrite.

This PR's welcome() change instead removes the first-run auto-guide entirely, which conflicts with (and would regress) that onboarding flow.

The one piece still worth keeping — the config.SourcePath() == "" guard on the ErrUnknownModel recovery in chatREPL — is carried forward in #2908.

Thanks for chasing this down. 🙏