fix(tui): defer --model validation until providers load

[kagura-agent]

Issue for this PR

Closes #23270

Type of change

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

What does this PR do?

Fixes --model flag being rejected at TUI startup because model validation runs before provider metadata loads.

Two problems:

1. Race condition: app.tsx onMount calls model.set() → isModelValid() checks sync.data.provider which is still empty → shows "not valid" toast for a valid model.

2. Priority override: Even when validation passes, the agent's configured model overwrites the CLI selection because the agent-change createEffect fires after mount.

Fix: Replace the eager onMount validation with a reactive cliOverride signal in local.tsx that resolves once providers load and takes highest priority in the model resolution chain. The agent-change effect skips auto-override when a CLI override is active.

How did you verify your code works?

Traced the full execution path: sync.tsx bootstrap (async provider fetch) → local.tsx model resolution (createEffect reactivity on sync.data.provider) → app.tsx mount lifecycle. Confirmed SolidJS createEffect re-evaluates when provider data arrives, and cliOverride correctly participates in getFirstValidModel priority chain.

Screenshots / recordings

N/A (logic-only change, no UI changes)

Checklist

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

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