Observability skills in Agent Builder

**What the feature is (as Title)**
A predefined set of Observability-specific skills registered on the Elastic Agent (Agent Builder), replacing the
current custom Observability AI Assistant.

**Value proposition**
Brings the observability capabilities of the existing AI Assistant into the unified Agent Builder experience, so users
get a single entry point for AI-assisted work across Elastic instead of solution-specific assistants. This unlocks
Agent Builder as the default AI experience in 9.4 and sets up a deprecation path for the standalone Observability AI
Assistant.

**Expected outcome**
- Users interact with the Elastic Agent and it transparently invokes observability skills (investigations,
visualizations, summaries, comparisons, etc.) when relevant.
- Skills are backed by evals that verify correct activation, tool selection, and result quality.
- Agent Builder becomes the default observability AI experience in 9.4, with the legacy AI Assistant on a defined
deprecation schedule.