M27.1: Add ExpertiseLevel and InteractionMode config types

[bug-ops]

Parent: #590

Scope

Add two new enums and extend AgentConfig in zeph-core/src/config/types.rs:

#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Deserialize, Serialize)]
#[serde(rename_all = "lowercase")]
pub enum ExpertiseLevel {
    Beginner,
    #[default]
    Intermediate,
    Expert,
}

#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Deserialize, Serialize)]
#[serde(rename_all = "lowercase")]
pub enum InteractionMode {
    #[default]
    Assistant,
    Coding,
    Automation,
}

Extend AgentConfig:

pub struct AgentConfig {
    pub name: String,
    pub max_tool_iterations: usize,
    pub summary_model: Option<String>,
    #[serde(default)]
    pub expertise_level: ExpertiseLevel,
    #[serde(default)]
    pub interaction_mode: InteractionMode,
}

Acceptance Criteria

• Enums with serde roundtrip (serialize/deserialize TOML)
• Default values: intermediate + assistant
• Backward compatible: existing configs without these fields still parse
• Re-export from zeph-core public API
• Unit tests for serde roundtrip and Default impl