M27.5: CLI flags --mode and --expertise

[bug-ops]

Parent: #590
Blocked by: M27.1

Scope

Add CLI flags to override config values for the session:

zeph --mode coding --expertise expert

Implementation

1. Add clap args in src/main.rs CLI struct
2. Override config values after loading TOML (CLI > config > default)
3. Follow existing pattern for --vault, --vault-key etc.

Acceptance Criteria

• --mode accepts assistant/coding/automation
• --expertise accepts beginner/intermediate/expert
• Invalid values produce clear clap error
• Config TOML values used when flags not provided
• Flags override config values