Prevent trace export filename collisions

[Gautam Sharma (GautamSharma99)]

Summary

Closes #227

Trace export now detects duplicate destination paths before writing any trace files, preventing silent truncation and misleading export counts.

What changed

• Resolve every root trace's filename up front using the configured {trace_id} and {name} substitutions.
• Reject duplicate paths before child traces are queried or any output file is created.
• Include the conflicting path and trace IDs in the actionable error; recommend {trace_id} or another unique placeholder.
• Reuse the same path resolver during the write phase so collision checking and file creation cannot diverge.

This preserves the existing default {trace_id}.jsonl behavior while making constant and non-unique {name} patterns fail safely instead of overwriting earlier exports.

Tests

• Constant and same-name patterns are verified to collide.
• {trace_id} patterns are verified to remain unique.
• go test ./...
• go test -race ./internal/cmd
• go vet ./...
• make build