fix(policies): require non-empty applies_to and fix the set-match example and coverage wording

[vishkaty]

Problem

Three small policies[] corrections found while reading the merged policies capability:

1. applies_to: [] is undefined. An empty array is schema-valid (no minItems on policy.json) but matches none of the three defined targeting forms (singular, set, omitted). Response-wide scope is expressed by omitting applies_to, so an empty array has no defined meaning.
2. The Set-match example matches nothing. overview.md filters on $.products[?@.category=='electronics'], but the product schema defines categories (array of category objects) and tags (array of strings), not category, so the canonical example selects the empty set against a schema-conformant product.
3. The coverage definition is circular. "the matched node's Normalized Path is a prefix of the target node's" is self-referential (the target matches a set, not a single node); it is the covered node whose path is prefixed.

Fix

• policy.json: add minItems: 1 to applies_to (omit the field for response-wide scope).
• overview.md: change the example filter to $.products[?@.tags[?@=='electronics']], a field products actually carry.
• overview.md: reword the coverage sentence to "a prefix of that node's Normalized Path".

Verification

No example in docs/ or source/ uses "applies_to": [], so minItems: 1 breaks nothing (confirmed by grep and a full validate_examples.py run: 282 passing, unchanged). ucp-schema lint source/ passes. The new JSONPath is valid RFC 9535.