#929 serve: log trace output directory on startup

Summary

Logs the resolved trace output directory when the trace consumer starts.

This makes session tracing discoverable in altimate serve by surfacing where trace files are being written at startup.

Changes

• Added TraceConsumer.getTraceDirectory()
• Logged the resolved trace directory after loadConfig() completes
• Only logs when a FileExporter is configured

Example log output:

[tracing] session traces directory=/path/to/traces

Validation

• bun run typecheck
• Verified typecheck passes successfully
• No behavior changes outside tracing startup logging

Fixes #916

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Summary by cubic

On startup, altimate serve now logs the resolved session trace directory so you can quickly find where trace files are written. The log appears only when a FileExporter is configured.

Summary by CodeRabbit

• New Features
  • Enhanced trace observability with improved startup logging that displays the configured trace output directory location when available.