P2: Add estimate_workflow tool for preflight cost/time projections

[Neftedollar]

Parent epic: #236

Problem

Clients cannot estimate likely runtime/cost before triggering expensive workflows.

Scope

• Add estimate_workflow tool using historical metrics + configured ceilings.
• Return confidence band and assumptions.
• Provide fallback when historical data is sparse.

Acceptance Criteria

• Tool returns structured estimate (cost, duration, confidence, basis).
• Estimates are available without starting a job.
• Documented caveats for low-signal workflows.

Non-goals

• Hard guarantees of runtime/cost.