refactor(runtime): separate inference access from reachability

[jdchawla29]

Summary

• model hosted inference access separately from workspace network reachability
• bind gateway-backed inference into isolated workspaces without exposing it as a general HUD API key
• keep managed CLI agents outside the Harbor harness boundary
• route scoped inference directly through the runtime-owned connection contract

Validation

• 735 focused agent, eval, environment, capability, client, and CLI tests passed
• repository CI formatting check passed
• repository CI Ruff check passed
• repository CI full-extra type check passed

Stack

Layer 2 of 4.

Depends on #624.
Next layer: #627.