feat: generic model selection via ACP session/set_model

[ironerumi]

Summary

Add generic model selection via ACP session/set_model that works across
any ACP agent advertising available models (Droid, Gemini CLI, Codex, etc.),
not tied to any agent's _meta conventions.

Motivation

Issue #49 covers model passthrough for Claude via _meta.claudeCode.options.model.
This is agent-specific. Agents like Droid, Gemini CLI, and Codex support
session/set_model as a first-class ACP method but don't use _meta.claudeCode.

A generic path is needed so model selection works uniformly across all agents.

Scope

• --model <id> flag at session creation → calls session/set_model after
  session/new when agent advertises available models
• acpx <agent> set model <id> during a session → routes through
  session/set_model instead of session/set_config_option
• Model state tracking, caching, and status enrichment
• Graceful degradation: agents without model support skip silently;
  ACP-coded rejections produce clear error messages
• Model cache only updated on successful set (not on rejection)

Related

• feat: add --model and --allowed-tools flags for agent passthrough #49 — Claude-specific model passthrough (complementary, not replaced)