docs: complexity routing config reference, --init wizard, live testing

[bug-ops]

Parent epic: #2141

Phase 6: Documentation + testing

Tasks

• Config reference in docs/src/ for [llm.complexity_routing] section
• Update --init wizard to include complexity routing setup (triage provider selection, tier mapping)
• Live testing with multi-tier provider pool (Ollama + Haiku + Sonnet)
• Test scenarios: simple greeting, medium coding task, complex architecture question, expert research
• Cross-mode testing: CLI + TUI
• Update CHANGELOG.md

Key files

• docs/src/ (config reference)
• crates/zeph-core/src/init.rs (wizard)
• .local/testing/ (test session records)

Acceptance criteria

• --init wizard offers complexity routing configuration
• Config reference documents all fields with examples
• Live test confirms correct tier routing across 4 complexity levels