Follow-up to #2336 (umbrella) / PR #2463 (partial sweep). Covers Area 5 deferred from that PR.

Scope

handleDirectiveOnly, applyInlineDirectivesFastLane, and their params carry aliasIndex, allowedModelKeys, allowedModelCatalog, resetModelOverride, defaultProvider, defaultModel, initialModelLabel, formatModelSwitchEvent. After #2335 these are always empty/trivial because createModelSelectionState is gone, but the whole directive-handling model-switching path is still wired.

• src/auto-reply/reply/directive-handling.params.ts — delete the model-state fields from HandleDirectiveOnlyCoreParams.
• src/auto-reply/reply/directive-handling.impl.ts — delete model-switching logic and unused params.
• src/auto-reply/reply/directive-handling.fast-lane.ts — delete destructures and handleDirectiveOnly call wiring.
• src/auto-reply/reply/get-reply-directives-apply.ts — delete the inline empty-default directiveModelState compat shim (introduced in gut(auto-reply): remove dead thinking-level resolution plumbing (#2334) #2335) and remaining aliasIndex / initialModelLabel / formatModelSwitchEvent threading.

AC

• All four files simplified; no empty-default compat shims remain.
• pnpm check + pnpm test pass.
• No user-facing behavior change (model-switching was already no-op post-gut(auto-reply): remove dead thinking-level resolution plumbing (#2334) #2335).

Context

• Parent: gut(auto-reply): finish thinking-level and model-selection sweep — follow-up to #2335 #2336
• Sibling shipped: PR gut(auto-reply): partial thinking-level + model-selection sweep — Areas 1+2+4 of #2336 #2463