Observability: trace clustering/search over historical traces #29

Split from #4 (LLM Observability Service). Depends on the base Phoenix tracing slice landing first.

Search/cluster over historical traces stored in Phoenix — this is the piece that matters for the tool-mining idea (finding patterns in which tools get called, for what kinds of queries, across many past conversations).

Scope not yet decided: whether this uses Phoenix's own UI/API as-is, or needs a thin backend integration (e.g. an admin screen linking into specific trace views). Needs its own design pass once the base tracing slice (#4) is in and we can see what Phoenix's native search actually supports self-hosted.

Out of scope for this issue: the base Phoenix service, OTel instrumentation of the chat agent, and eval hooks (see the separate eval-hooks issue).