fix(observability): map Claude Code user_prompt -> input.value so Langfuse Input isn't null (#59)

Claude Code emits the prompt as the span attribute 'user_prompt' (on
claude_code.interaction), but Langfuse reads observation Input from the OpenInference
'input.value' convention -> the Input field showed null while the prompt sat only in the
raw-attributes panel. Add an OTTL transform mapping user_prompt -> input.value.

Assistant OUTPUT text is NOT emitted on the trace signal (Claude Code sends it via the
logs signal, which Langfuse trace ingestion does not consume) -> output shows
tokens/cost/finish_reason only; documented inline.

Token usage + cost already work (the gen_ai.usage.* transform on main); the recent
'all null' was the stack being down after reboot (4 containers were removed and did not
auto-recreate) - brought back up, restart: unless-stopped carries them across reboots.

Verified: fresh trace -> observation usage {input:10,output:59,cache...} cost $0.109;
claude_code.interaction Input = the prompt text.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: AndreJorgeLopes

docker/otel-collector-config.yaml

@@ -25,6 +25,12 @@ processors:
           - set(attributes["gen_ai.usage.output_tokens"], attributes["output_tokens"]) where attributes["output_tokens"] != nil
           - set(attributes["gen_ai.usage.cache_read_input_tokens"], attributes["cache_read_tokens"]) where attributes["cache_read_tokens"] != nil
           - set(attributes["gen_ai.usage.cache_creation_input_tokens"], attributes["cache_creation_tokens"]) where attributes["cache_creation_tokens"] != nil
+          # Content: Claude Code emits the prompt as `user_prompt` (on claude_code.interaction);
+          # Langfuse reads observation Input from OpenInference `input.value`. Map it so the
+          # Input field shows the prompt instead of null. (No completion-text attr exists on
+          # the trace signal — Claude Code emits assistant output only via the logs signal,
+          # which Langfuse trace ingestion does not consume; output shows tokens/cost only.)
+          - set(attributes["input.value"], attributes["user_prompt"]) where attributes["user_prompt"] != nil
 
 exporters:
   # Langfuse appends /v1/traces to this base path.