HYBIM-898 Update migration tool README(new - duplicate of PR #224)

[ridhima-splunk]

Updated migration guide with domain entity renames (Metrics → Evaluators, Log Streams → Agent Streams)

• Added §3.3 Metric / Evaluator class rename table: GalileoMetric → SplunkAOEvaluator, GalileoMetrics → SplunkAOEvaluators, and lower-level renames

Metric → Evaluator, LlmMetric → LlmEvaluator, LocalMetric → LocalEvaluator, CodeMetric →     
 CodeEvaluator, BuiltInMetrics → BuiltInEvaluators, MetricSpec → EvaluatorSpec, LocalMetricConfig → LocalEvaluatorConfig

with import diff examples

• Added §3.3a Log Stream / Agent Stream rename table: LogStream → AgentStream, LogStreams → AgentStreams, method renames on AgentStream (get_metrics(), enable_metrics()) and Project (create_log_stream(), list_log_streams(), .logstreams), with import and usage diff examples
• Updated §2.2 sub-module imports: galileo.metric → splunk_ao.evaluator
• Updated checklist with all new rename items

Note:

• §5.3 on-disk config path (~/.galileo/ → ~/.splunk/) will be addressed in PR#223