Issue #3586270: Document how the Tag scope works

Description

Fleshes out the Tag scope section in docs/features/scopes.md, which was too thin to explain how the scope actually works (per @ultimike in the issue).

Adds:

• The end-to-end workflow: create vocabulary terms → tag context items → subscribe an agent to the same tags → tagged items score higher when that agent runs.
• An explicit clarification that the Tag scope works through agent subscriptions, not contextual detection — it does not read a taxonomy reference field off the entity being processed. Points to the Target entity and Entity bundle scopes for entity-driven matching, and notes hook_ai_context_scope_values_alter() as the customization seam.

No code changes; documentation only.

Closes #3586270 (closed).

Testing instructions

Read the updated Tag section at docs/features/scopes.md (or the rendered docs site) and confirm it matches the behavior of AiContextScopeTag — subscription-based scoring with doGetCurrentValue() returning NULL (no contextual detection).