feat(acp): auto-populate available_models from configured LLM providers

[bug-ops]

Part of epic #978.

Problem

available_models in AcpConfig is a static list in config.toml that the user must
maintain manually. As seen in Zed's model picker (built-in agents show all provider
models dynamically), users expect to see all models that Zeph has access to — grouped
by provider — without manual configuration.

Current flow:

config.toml [acp] available_models = ["claude-opus-4-6", "gpt-4o"]
    → AcpAgentState::available_models
    → SessionModelState { available_models }
    → Zed model picker

Desired flow:

LlmOrchestrator::list_models() (per-provider)
    → deduplicated, sorted list
    → SessionModelState { available_models }
    → Zed model picker (shows all configured models automatically)

Design

1. Add list_models() -> Vec<String> method to LlmProvider trait in
   zeph-llm/src/provider.rs — returns model IDs this provider supports.
2. Implement for ClaudeProvider, OpenAiProvider, OllamaProvider:
   • Claude/OpenAI: return static list of known models (from config model field + known variants)
   • Ollama: query local API GET /api/tags to list installed models
3. In LlmOrchestrator (zeph-llm): aggregate list_models() across all providers.
4. In src/acp.rs: if config.acp.available_models is empty, populate from
   orchestrator.list_models() at startup.
5. Keep manual config.toml override working (non-empty list takes precedence).

Acceptance criteria

• When [acp] available_models = [], Zed model picker shows all models from
  configured providers automatically
• Ollama models reflect currently installed models at startup
• Static override (available_models = [...] in config) still works
• Model IDs shown in picker match IDs accepted by /model <id> command

Files

• crates/zeph-llm/src/provider.rs — add list_models() to trait
• crates/zeph-llm/src/claude.rs, openai.rs, ollama.rs — implement trait method
• crates/zeph-llm/src/orchestrator.rs — aggregate across providers
• src/acp.rs — auto-populate if config list is empty
• crates/zeph-core/src/config/types.rs — document new auto-discovery behaviour