Enhancement: show routing preview in `nyx tasks inspect`

[samkeen]

Context

Surfaced during review of user story US-01, Story 1-C scenario "Inspect a task with no episodes" on 2026-04-10. Tracked here so it isn't lost.

Proposal

When nyx tasks inspect <id> runs against a todo task, additionally show where the task would be routed if executed right now. Something like:

Routing preview:
  backend:  claude-cli
  endpoint: anthropic/claude-opus-4-6
  fallback: openrouter/anthropic/claude-sonnet-4-6
  matched:  [[routes]] #2 (type = "coding", priority = ">5")

The Model Router already has the logic (§3c); inspect would just call it in a read-only / preview mode that returns the ModelAssignment without triggering any side effects.

Why it matters

• Operators queueing tasks overnight want to confirm "this task will actually run where I expect" before they start a shift — not find out in the morning report.
• Makes routing config bugs visible at queue time rather than at dispatch time (complements the strict type validation pinned in §7 decision 17).
• Useful debugging tool when routes get complex.

Scope notes

• Read-only: the preview must not mutate any state, emit any execution events, or hit backend health_check() endpoints (those may be expensive or rate-limited).
• Should also work for --json output, as a routing_preview object on the task record.
• Out of scope for US-01 scenario review — deliberately deferred to avoid scope creep. Worth scheduling when the Model Router lands (M2) since that's when the feature becomes implementable.

Not blocking

Milestone: post-M2 (needs the Router to exist).