Add [all] sentinel to bc-version; apply to version-agnostic knowledge

Most of the corpus — FindSet/SetLoadFields/CalcFields patterns, permission
sets, SingleInstance codeunits, DataClassification, IsolatedStorage,
transaction scope, SecretText — describes BC platform behaviour that is
identical across supported versions. The seed [26..28] range on every
file implied a version-specificity the content does not actually have,
and there was no way to express "applies to every version" in the
schema the way [w1] and [all] already do for countries and
application-area.

Extend the v1 schema with a universal sentinel for bc-version, parallel
to the sentinels already defined for the other dimensions:

  bc-version: [all]         # applies to every BC version

[all] is mutually exclusive with explicit versions. Range shorthand
([26..28]) and explicit lists ([26, 27, 28]) continue to work for files
genuinely tied to a version-gated API or deprecation.

Update read.md (field definition, matching semantics, partial-context
rule), write.md (default to [all], use ranges only with a concrete
reason), README.md (frontmatter example), and the CI validator. All
forty existing knowledge files and the three action skills convert to
[all]; none of the current content is version-gated. Validator passes.

microsoft/skills/al-performance-review.md

@@ -6,7 +6,7 @@ title: AL performance review
 description: Reviews AL source changes against performance guidance from BCQuality.
 inputs: [pr-diff, file-path]
 outputs: [findings-report]
-bc-version: [26..28]
+bc-version: [all]
 technologies: [al]
 countries: [w1]
 application-area: [all]