feat(traces): compare two AgentCore traces from the CLI

[OrenAshkenazy]

Problem

agentcore traces list finds traces and agentcore traces get exports raw
CloudWatch records, but there is no human-friendly way to compare two agent
invocations.

Users testing runtime versions or named endpoints must manually download JSON,
inspect CloudWatch’s incomplete single-span view, and calculate latency/token
differences themselves. This makes performance regression testing impractical.

Proposed command

agentcore traces compare <baseline-trace-id> <candidate-trace-id> \
  --runtime <runtime-name> \
  [--since <time>] \
  [--until <time>] \
  [--json]

The command should fetch both traces directly from the existing CloudWatch
trace/span sources. It must not require users to export JSON files first.

Example output:

Trace comparison: baseline abc123 → candidate def456

Metric                      Baseline     Candidate                   Delta
End-to-end latency            10.00s         6.33s         -3.66s (-36.6%)
LLM latency                    7.36s         3.67s         -3.69s (-50.1%)
Tool latency                   2.26s         2.58s         +0.32s (+14.3%)
LLM calls                          2             2                0 (0.0%)
Tool calls                         1             1                0 (0.0%)
Input tokens                 151,266         2,848       -148,418 (-98.1%)
Output tokens                    595           300           -295 (-49.6%)
Total tokens                 151,861         3,148       -148,713 (-97.9%)

Baseline model(s): us.anthropic.claude-haiku-4-5-20251001-v1:0
Candidate model(s): us.anthropic.claude-haiku-4-5-20251001-v1:0

Required behavior

• Resolve the runtime/project target using the existing traces command
  conventions.

• Fetch structured span records from CloudWatch directly.

• Use the POST /invocations server span for end-to-end latency when present.
  If absent, fall back to earliest span start through latest span end and label
  that fallback clearly.

• Report LLM and tool time separately using existing GenAI span attributes.

• Avoid double-counting nested provider spans. For example, a Strands internal
  LLM span and its nested Bedrock client span represent the same model call.

• Include LLM/tool call counts and input/output/total token counts when
  available.

• Show absolute and percentage deltas; handle a zero baseline safely.

• Provide stable machine-readable output with --json.

• Fail clearly if either trace cannot be found or has no usable timed spans.

• Do not claim a “critical path” calculation: CloudWatch trace parent/child
  relationships may be incomplete.

• Show comparability warnings when observable characteristics differ, such as
  LLM-call count, tool-call count, or token usage. The CLI cannot prove that

Scope

This is a trace-analysis and latency-comparison feature. It is independent of:

• runtime endpoint selection for agentcore invoke
• AgentCore Gateway A/B-test infrastructure
• SigNoz or other third-party OTEL observability backends

Acceptance Criteria

• agentcore traces compare works for two trace IDs from the same runtime.

• It produces the table above or equivalent concise terminal output.

• --json returns documented structured data suitable for CI benchmarking.

• Unit tests cover root-span selection, fallback timing, nested-span
  de-duplication, token aggregation, deltas, warnings, and missing-trace
  errors.

• Existing traces list and traces get behavior remains unchanged.

Additional Context

No response