fix(tui): handle composed Option-D in model picker

[bwachman]

Summary

• The model picker's Alt+D set-default shortcut now also recognizes the composed ∂ glyph some terminals (e.g. iTerm with a US layout) emit for Option+D instead of a modifier-flagged d, mirroring the existing Option+A handling for the add-provider shortcut.
• Outside the model picker, ∂ still types as ordinary filter text.
• On the "(no matches)" filter sentinel, both the real Alt+D chord and the composed ∂ are now consumed as a no-op rather than falling through, so an empty model id can never reach the default-setting callback.

Verification

• bun run typecheck, bun run build, and bun run test pass (7747 pass, 0 fail)
• New/updated tests in product-host.test.ts cover composed-∂ set-default, composed-∂ as plain filter text outside the model picker, and the no-matches-sentinel case for both the real chord and the composed glyph

Fixes CL-8635

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[github-actions]

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[linear-code]

CL-8635

[bwachman]

I have read the CLA Document and I hereby sign the CLA

[TheGreatAxios]

Approve — the composed Option-D handling is correct and well-scoped.

What I verified:

• The dispatch chain works end to end: composed ∂ (no modifiers) bypasses the filter only when the set-default hint is showing in the model picker, then flows through the overlay action into the existing set-default handler (55/55 tests in product-host.test.ts pass, including the 3 new/updated cases).
• No input gets swallowed: outside the picker, ∂ stays ordinary filter text, and bare ASCII d still requires the modifier — typing is unaffected. The single-entry glyph set is complete since ∂ has no uppercase variant.
• The sentinel change (swallow instead of fall-through on no-matches) fixes real prompt pollution — previously the chord leaked into the prompt. The Alt+D expectation flip is the same fix on the modifier path, consistent.
• Architecture is right: normalization lives at the overlay-action layer rather than global input (where ∂ is legitimate text elsewhere), mirrors the existing Option-A precedent, and the composition is terminal-generic rather than iTerm-specific.

Two follow-ups, neither blocking:

• Alt+F (favorite) has the same composed-glyph gap on the sentinel — worth a ticket if anyone hits it there.
• Each new chord repeats the glyph-allowlist discovery cycle; fine as-is given the scoped-predicate pattern.