[P2.1-T01] Add PolicyMode enum

[RichardHightower]

Task: Add PolicyMode enum

Phase: 2.1 - Core Governance
Estimated: 0.5 day
File: cch_cli/src/models/mod.rs

Description

Create the PolicyMode enum to support enforce, warn, and audit modes for rules.

Acceptance Criteria

• Create PolicyMode enum in models/mod.rs
• Values: Enforce, Warn, Audit
• Implement Default trait (default = Enforce)
• Implement Deserialize for YAML parsing (case-insensitive)
• Implement Serialize for JSON output
• Add unit tests for parsing

Code Reference

#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum PolicyMode {
    #[default]
    Enforce,
    Warn,
    Audit,
}

Tests Required

• Parse "enforce", "warn", "audit" (case-insensitive)
• Default to Enforce when absent
• Serialize to lowercase in JSON output

[RichardHightower]

Completed in PR #72