Observability: full run tracing and inspection UI

[fusion94]

PRD: FR-13 (expanded). User story US-5.

Expand the basic trace (#8) into full, navigable run inspection.

Scope

• Rich trace view: model calls, tool calls, inputs/outputs, decisions, timing
• Filter/search across runs and sessions
• Link traces to cost accounting (Token and cost accounting per run and session #20)

Done when

The operator can fully reconstruct and navigate any past run.