fix(#2544): exit 1 on missing key in config-get (#2588)

The configGet query handler previously threw GSDError with
ErrorClassification.Validation, which maps to exit code 10. Callers
using `if ! gsd-sdk query config-get key; then fallback; fi` could
not detect missing keys through the exit code alone, because exit 10
is still truthy-failure but the intent (and documented UNIX
convention — cf. `git config --get`) is exit 1 for absent key.

Change the classification for the two 'Key not found' throw sites to
ErrorClassification.Execution so the CLI exits 1 on missing key.
Usage/schema errors (no key argument, malformed JSON, missing
config.json) remain Validation.

Closes #2544

sdk/src/query/config-query.ts

@@ -104,12 +104,14 @@ export const configGet: QueryHandler = async (args, projectDir, _workstream) =>
   let current: unknown = config;
   for (const key of keys) {
     if (current === undefined || current === null || typeof current !== 'object') {
-      throw new GSDError(`Key not found: ${keyPath}`, ErrorClassification.Validation);
+      // UNIX convention (cf. `git config --get`): missing key exits 1, not 10.
+      // See issue #2544 — callers use `if ! gsd-sdk query config-get k; then` patterns.
+      throw new GSDError(`Key not found: ${keyPath}`, ErrorClassification.Execution);
     }
     current = (current as Record<string, unknown>)[key];
   }
   if (current === undefined) {
-    throw new GSDError(`Key not found: ${keyPath}`, ErrorClassification.Validation);
+    throw new GSDError(`Key not found: ${keyPath}`, ErrorClassification.Execution);
   }
 
   return { data: current };