feat(llm): complexity-based triage routing for multi-provider pool

[bug-ops]

Summary

Implements complexity-based pre-inference triage routing for the multi-provider LLM pool (closes #2141).

Before each LLM call, a configurable classifier provider evaluates input complexity and routes to a tier-specific provider:

• Simple → fast/cheap model (e.g. Haiku, qwen3:1.7b)
• Medium → default model
• Complex → smart model (e.g. Sonnet, GPT-4o)
• Expert → expert model (e.g. Opus, o1)

Changes

• crates/zeph-llm/src/router/triage.rs (new) — ComplexityTier, TriageRouter, TriageMetrics
• crates/zeph-llm/src/any.rs — AnyProvider::Triage variant + all match sites
• crates/zeph-config/src/providers.rs — LlmRoutingStrategy::Triage, ComplexityRoutingConfig, TierMapping
• crates/zeph-core/src/bootstrap/provider.rs — build_triage_provider()
• crates/zeph-core/src/agent/tool_execution/native.rs — status indicator
• src/init.rs — --init wizard step

Config

[llm.complexity_routing]
enabled = true
triage_provider = "fast"
bypass_single_provider = true
triage_timeout_secs = 5

[llm.complexity_routing.tiers]
simple = "fast"
medium = "default"
complex = "smart"
expert = "expert"

Key design decisions

• TriageRouter implements LlmProvider directly via Box::pin to break the recursive AnyProvider type cycle
• Classifier call wrapped in tokio::time::timeout; regex fallback on malformed JSON output
• Context-window auto-escalation skips gracefully when provider returns None
• last_provider_idx (AtomicUsize) tracks last-used tier for correct cost/token reporting
• set_status_tx() propagates to all tier providers — streaming indicators work during inference
• bypass_single_provider compares config entry names (not runtime type names) for heterogeneous pools
• All metrics use AtomicU64 — no Mutex in async paths

Tests

18 new unit tests: tier routing, timeout fallback, context-window escalation (incl. None case), metrics counters, config TOML round-trip, LlmRoutingStrategy::Triage deserialization.

Total: 6443 passed (was 6432, +11 net after deduplication).

Deferred (follow-up issues)

• TUI metrics panel wiring (metrics available via TriageRouter::metrics())
• Interactive --init wizard step (config schema added, literal updated)
• Prompt injection hardening (system message prefix in triage call)
• chat_stream/chat_with_tools async delegation tests