Add config-based LLM provider selection

[bug-ops]

Description

Allow selecting between Ollama and Claude via configuration.

Parent: #4

Branch

feat/m3/provider-selection

Files

• crates/zeph-core/src/config.rs — add cloud LLM config, provider enum
• crates/zeph-core/src/agent.rs — construct provider from config
• src/main.rs — provider factory
• config/default.toml — add cloud section

Config

[llm]
provider = "ollama"  # or "claude"
base_url = "http://localhost:11434"
model = "mistral:7b"

[llm.cloud]
model = "claude-sonnet-4-5-20250929"
max_tokens = 4096
# API key via ZEPH_CLAUDE_API_KEY env var

Acceptance Criteria

• llm.provider = "ollama" uses OllamaProvider (default)
• llm.provider = "claude" uses ClaudeProvider
• ZEPH_LLM_PROVIDER env var overrides config
• Missing API key for Claude: clear error message

Depends On

#19 (Claude backend), #18 (Memory integration)