feat: add dedicated API endpoint for symbol search (find command)

[jonathanpopham]

Context

The find command (internal/find/handler.go) does substring matching across graph nodes to find symbols and their callers/callees. Like focus, there is no dedicated API endpoint — it uses the generic graph.

Current state

• supermodel find <symbol> uses /v1/graphs/supermodel, iterates all nodes, does case-insensitive substring match
• Shows callers, callees, and source file for each match

Proposal

Either:

1. API-side: Add POST /v1/analysis/find or GET /v1/search with indexed symbol lookup, fuzzy matching, and caller/callee expansion
2. CLI-side: Keep using the graph (acceptable for now since substring search on graph nodes works)

Blocked on

• Decision on whether this warrants an API endpoint or should remain client-side