feat: Context window % visualization on web dashboard

[aviraldua93]

Problem

When running multi-agent workflows, developers have no visibility into how much of each agent's context window is consumed. Context overflow is flagged as RISK-008 in the conductor design docs, but the dashboard doesn't show utilization.

Agents can silently hit context limits and degrade — especially during longer workflows with context accumulation mode, iterative loops, or parallel research patterns.

Proposed Solution

Show real-time context window percentage on the web dashboard:

• Agent node: Progress bar (green <70%, amber 70-89%, red 90%+, pulse at 100%)
• Detail panel: Context row in MetadataGrid (e.g., \17,862 / 200,000 (8.9%))
• Backend: Model context window lookup tables for Claude and Copilot providers
• Events: New \context_window_used\ and \context_window_max\ fields on agent lifecycle events

Implementation

PR: #56

Addresses

• RISK-008: Context token overflow (from \docs/projects/initial-implementation/copilot-conductor.design.md)