M27.3: Refactor build_system_prompt() to use mode and expertise

[bug-ops]

Parent: #590
Blocked by: M27.1, M27.2

Scope

Refactor build_system_prompt() in zeph-core/src/context.rs:

Before

pub fn build_system_prompt(
    skills_prompt: &str,
    env: Option<&EnvironmentContext>,
    tool_catalog: Option<&str>,
    native_tools: bool,
) -> String

After

pub fn build_system_prompt(
    skills_prompt: &str,
    env: Option<&EnvironmentContext>,
    tool_catalog: Option<&str>,
    native_tools: bool,
    expertise: ExpertiseLevel,
    mode: InteractionMode,
) -> String

Assembly order:

1. Persona block (from mode)
2. Philosophy block (from expertise)
3. Tool use block (native vs legacy — unchanged)
4. Guardrails block (from expertise x mode)
5. Environment context
6. Tool catalog
7. Skills prompt

Callers to update

• Agent::new() in agent/mod.rs
• reload_skills() in agent/mod.rs

Acceptance Criteria

• Signature updated with new params
• All existing callers updated
• Existing tests updated to pass new params
• New tests: verify prompt content varies by mode/expertise combination
• Prompt caching in Claude provider still works (no regression)