Add model switching to the lean TUI

[rumpl]

Add /model support to the lean TUI and share fuzzy model search between the lean and full TUIs.

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

One medium-severity issue found in the model picker filtering logic introduced by this PR.

[docker-agent]

[MEDIUM] Custom-model override replaces all fuzzy matches when query contains "/" but isn't an exact match

In filterModels(), the comment says "show Custom option as well as matches" but the code replaces d.filtered entirely instead of appending to it:

// A provider/model query is an explicit reference rather than a fuzzy
// search when it does not identify an available choice.
if isCustomQuery && !slices.ContainsFunc(d.models, func(model runtime.ModelChoice) bool {
    return strings.EqualFold(model.Ref, query) || strings.EqualFold(model.Provider+"/"+model.Model, query)
}) {
    d.filtered = []runtime.ModelChoice{{   // ← replaces, does not append
        Name: "Custom: " + query,
        Ref:  query,
    }}
}

The condition checks d.models (all models, for an exact match) — not d.filtered (fuzzy results already computed). So for a partial query like openai/gpt, modelpicker.Filter may have returned several relevant matches, but since "openai/gpt" is not an exact EqualFold match for any model's Ref or Provider+"/"+Model, those fuzzy results are discarded and only "Custom: openai/gpt" is shown.

Suggested fix: append the custom entry to the existing fuzzy results rather than replacing them, and only show the custom entry when fuzzy search returns nothing:

if isCustomQuery && !slices.ContainsFunc(d.models, func(model runtime.ModelChoice) bool {
    return strings.EqualFold(model.Ref, query) || strings.EqualFold(model.Provider+"/"+model.Model, query)
}) {
    customEntry := runtime.ModelChoice{
        Name: "Custom: " + query,
        Ref:  query,
    }
    if len(d.filtered) == 0 {
        d.filtered = []runtime.ModelChoice{customEntry}
    } else {
        d.filtered = append(d.filtered, customEntry)
    }
}

[rumpl]

Good catch. This was a real issue: partial provider/model queries could produce useful fuzzy matches, but the custom entry replaced them. The picker now appends the custom entry after the fuzzy results, and the test covers retaining a fuzzy match alongside the custom option. Fixed in fe27014.

[docker-agent]

Thanks for the quick fix! The refactoring looks solid — extracting the fuzzy search into a shared package is a nice improvement, and appending the custom entry after fuzzy matches solves the replacement issue perfectly. The updated test also correctly validates that both fuzzy matches and the custom option are retained.