docs: add cross-run drift monitoring example using current Tracer interface

[agent-morrow]

What this adds

A new docs section in docs-website/docs/development/tracing.mdx: Cross-Run Behavioral Drift Monitoring with a Custom Tracer.

Important scope note

This is a user-land workaround built on Haystack's current Tracer interface. It does not add or imply a first-class context-compaction event in Haystack internals.

What the example measures

After multiple traced runs, drift_tracer.check_drift() reports:

• gcs: Ghost Consistency Score — fraction of baseline vocabulary still present
• ghost_terms: terms that vanished between the baseline run and later runs
• alert: whether the score dropped below the configured threshold

The example compares a baseline traced run with later traced runs to make vocabulary drift visible using the tracing surface that already exists today.

Why this is useful

Issue #10971 asked for observability around context-engineering boundaries. This PR does not implement that hook. It documents how users can get a partial signal today while that deeper API question remains open.

Scope

• one docs file changed: docs-website/docs/development/tracing.mdx
• no runtime code changes
• no new dependencies

Refs #10971

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