feat: auto model routing for SkillFlows

[RiteshTiwari1]

Addresses #48

Summary

Adds automatic model routing for SkillFlows. Each step is classified by
complexity at runtime and routed to the right model — lightweight tasks
(summarize / extract / classify / transform) run on a cheaper model, while
reasoning-intensive tasks (planning / decision-making / tool orchestration)
stay on the configured reasoning model. Reduces token usage and cost without
sacrificing quality on complex steps.

How it works

Model resolution per step, in priority order:

1. Explicit per-step model: in workflows/*.yaml
2. Per-skill model: in SKILL.md
3. Automatic classification via model.routing tiers
4. Fallback to the preferred model

Unknown / ambiguous tasks default to reasoning, so cost optimization never
silently degrades quality. Routing is opt-in — active only when a
model.routing block is present, so existing agents are unaffected.

Configuration (agent.yaml)

model:
  preferred: "openai:gpt-5-reasoning"
  routing:
    lightweight: "openai:gpt-4o-mini"
    reasoning:   "openai:gpt-5-reasoning"

Scope note

Routing is applied at the SkillFlow step level — where GitAgent runs
discrete tasks in sequence. Per-turn routing inside a single agent loop would
need deeper pi-agent-core changes; happy to follow up if that's preferred.