feat: trace the Memory write path

[PsiACE]

Feature description

Add bounded tracing for source capture and the Memory flush pipeline.

Problem and proposed solution

flush_memory currently covers extraction, embedding, and persistence without showing which stage completed or failed.

Possible trace structure:

powercontext capture_content_source
└── memory.capture

powercontext flush_memory
└── memory.flush
    ├── invoke_agent memory_extraction
    ├── embeddings <model>
    └── memory.commit

Record only bounded counts, latency, outcome, and error category. Do not record Source content, Memory content, prompts, responses, or vectors.

The PR should include an exported span tree or dashboard screenshot.

Alternatives considered

Keep one span for the complete flush operation.

Additional context

Part of #1214. Tracing remains optional and must not change Runtime behavior.

[Oxidaner]

Hi, I'm interested in this, please assign it to me. I think this issue can be completed after Issue 1241

[PsiACE]

@Oxidaner Thanks, I've already assigned this to you, but there's no need to wait for an update on 1241.

[Oxidaner]

Ok. I will develop it as soon as possible