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.

.github/scripts/validate_frontmatter.py

@@ -145,10 +145,19 @@ def is_non_empty_list_of_str(value: Any) -> bool:
     return isinstance(value, list) and len(value) > 0 and all(isinstance(v, str) and v for v in value)
 
 
-def expand_bc_version(value: Any) -> tuple[list[int] | None, str | None]:
-    """Return (expanded-list, error-message). One of the two is None."""
+def expand_bc_version(value: Any) -> tuple[list[int] | str | None, str | None]:
+    """Return (expanded, error-message). One of the two is None.
+
+    For the universal sentinel ["all"], `expanded` is the string "all".
+    Otherwise it is the expanded list of version integers.
+    """
     if not isinstance(value, list) or not value:
         return None, "must be a non-empty list"
+    # Case 0: universal sentinel
+    if len(value) == 1 and value[0] == "all":
+        return "all", None
+    if "all" in value:
+        return None, "'all' is mutually exclusive with explicit versions"
     # Case 1: all integers
     if all(isinstance(v, int) and not isinstance(v, bool) for v in value):
         if any(v <= 0 for v in value):
@@ -162,7 +171,7 @@ def expand_bc_version(value: Any) -> tuple[list[int] | None, str | None]:
             if start > end:
                 return None, f"range '{value[0]}' is not ascending"
             return list(range(start, end + 1)), None
-    return None, "must be a list of integers or a single-element range shorthand like [26..28]"
+    return None, "must be [all], a list of integers, or a single-element range shorthand like [26..28]"
 
 
 def headings_in_order(body: str) -> list[tuple[str, int]]: