Add Semantic Signal Details experience event field group

[AkankshaN]

Summary

Closes #2244

Adds a new experimental, system-generated field group, Semantic Signal Details, that captures a qualified semantic signal extracted from an unstructured source interaction (conversation, session, or document). Extends experienceevent. Non-breaking, additive-only change — no existing schemas are modified.

Motivation

Conversational/agentic experiences (chat, voice, document interactions) need a standard way to record derived semantic signals — e.g. intent or preference qualifiers scored by an extraction model — as system-generated telemetry on an ExperienceEvent.

Changes

• components/fieldgroups/agentic/semantic-signal-details.schema.json — new field group defining xdm:semanticSignal (marked meta:usereditable: false), with xdm:name, xdm:group, xdm:score, xdm:strength (hard enum: low/medium/high, documented via meta:enum), xdm:channel (reuses existing channels/channel datatype), xdm:sourceId, xdm:definitionVersion, xdm:extractionModelVersion. meta:status: experimental, meta:intendedToExtend: experienceevent.
• components/fieldgroups/agentic/semantic-signal-details.example.1.json — new example validating the schema.

Validation

• npm test — 2417 passing, 1 pending
• npm run lint — clean
• npm run validate — 58 failures, identical to master baseline (zero new failures)
• npm run incompatibility-check — clean

Breaking changes

None.

[Prashanthi-Ram]

Please also add to the fieldgroup. meta:abstract: true.

meta:abstract : true means you can't directly instantiate the JSON as an XDM schema
For example, you can't use classes, field groups, data types directly as XDM schema and load data against it. These are abstract components, and you only can refer them from a schema to create an instance
It is required on every global FG, classes, data types