[codex] Consolidate slash command boundaries

[roackb2]

Summary

• Move the slash command result contract into src/core/commands/slash/result-types.ts as SlashCommandResult.
• Remove the old LocalCommandResult type from chat and avoid compatibility aliases or re-exports.
• Make TUI local command autocomplete delegate to the core slash autocomplete helpers.
• Update command domain README language to describe the current implementation.

Why

This is M11.5-A from the domain-module consolidation pass. The command domain now owns command result semantics, and the TUI no longer carries duplicate autocomplete logic beside the core helper.

Validation

• yarn test:unit src/__tests__/unit/tui/local-commands.test.ts src/__tests__/unit/core/slash-commands.test.ts src/__tests__/unit/core/slash-command-modules.test.ts
• yarn eslint
• yarn typecheck