Add knowledge-backed AL development

Add read-only planning and repository-changing development skills so BCQuality
knowledge can guide features, bug fixes, refactors, upgrades, and maintenance
before the existing AL review gate runs. Track Microsoft Learn ingestion and
add development and BCApps-shaped guidance evaluation fixtures.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 638b66d2-9f06-4f60-8781-808709e1485c

microsoft/skills/review/al-interfaces-review.md

@@ -39,7 +39,7 @@ Narrow the relevant files to the subset that applies to the changes under review
 
 - The changed AL object names and types — especially `interface` objects, codeunits and enums declared with the `implements` keyword, and consumers that declare or assign an `Interface` variable.
 - The changed procedures and triggers, weighted toward factory or dispatch routines that resolve a variant to behaviour, setter-injection procedures that take an `Interface` parameter, and `case`-over-enum blocks that select between strategies.
-- Tokens extracted from the diff that relate to interfaces and enum-backed implementation (`interface`, `extends`, `implements`, `Implementation`, `DefaultImplementation`, `UnknownValueImplementation`, `enum`, `Extensible`, `Interface`, `case`, and the `case <enum> of` anti-pattern signal — a `case` over an enum value whose branches choose between variant computations).
+- Tokens extracted from the diff that relate to interfaces and enum-backed implementation (`interface`, `extends`, `implements`, `Implementation`, `DefaultImplementation`, `UnknownValueImplementation`, `enum`, `Extensible`, `Interface`, `Variant`, `is`, `as`, `case`, and the `case <enum> of` anti-pattern signal — a `case` over an enum value whose branches choose between variant computations).
 
 A file enters the candidate worklist when its `keywords` intersect the extracted tokens or its topic (derived from the index entry's `path`, `title`, and `description`) matches a changed object type. Read an article's full file — its `## Best Practice` / `## Anti Pattern` bodies — only after it makes the worklist; candidate selection uses the index alone.
 
@@ -54,6 +54,7 @@ The following targeted checks map diff signals to specific `interfaces` articles
 - `DefaultImplementation` used as the only fallback where a persisted ordinal may no longer match any declared enum value, or a persisted enum lacks `UnknownValueImplementation` on BC18 or later — `handle-unknown-enum-ordinals-with-unknownvalueimplementation`.
 - A method added directly to an interface that exists in the baseline, instead of adding a BC25+ interface that `extends` it or a versioned sibling for older targets — `extend-published-interfaces-dont-edit-them`.
 - A declared enum value with no `Implementation` and no enum-level `DefaultImplementation` — `set-defaultimplementation-on-enum`.
+- An `Interface` or `Variant` is cast with `as` to an optional extended interface without first establishing support with `is` — `guard-interface-casts-with-is`.
 
 For `set-defaultimplementation-on-enum`, inspect the complete containing enum before emitting. An enum-level `DefaultImplementation = <Interface> = <Codeunit>;` conclusively covers every declared value that omits its own `Implementation`; do not flag such a value and do not replace the intentional fallback with a per-value mapping.