docs: add per-ability isAvailable() to plan and spec

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

docs/superpowers/plans/2026-09-25-etappe-0-anbindung.md

@@ -670,10 +670,10 @@ Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>"
 **Interfaces:**
 - Consumes: `Access::currentUserMayUse()`, `Access::CAPABILITY` (Task 2).
 - Produces:
-  - `interface Ability { name(): string; label(): string; description(): string; inputSchema(): array; outputSchema(): ?array; annotations(): array; isDirect(): bool; permission(array $input): bool|\WP_Error; execute(array $input): mixed; }`
+  - `interface Ability { name(): string; label(): string; description(): string; inputSchema(): array; outputSchema(): ?array; annotations(): array; isDirect(): bool; isAvailable(): bool; permission(array $input): bool|\WP_Error; execute(array $input): mixed; }`
   - `Errors::forbidden(string $message): \WP_Error` (Code `lit_forbidden`), `Errors::invalidInput(string $message, array $fields = []): \WP_Error` (Code `lit_validation_failed`), `Errors::notFound(string $message): \WP_Error` (Code `lit_not_found`), `Errors::internal(): \WP_Error` (Code `lit_internal_error`).
   - `interface Integration { id(): string; isAvailable(): bool; /** @return list<Ability> */ abilities(): array; }`
-  - `IntegrationRegistry::__construct(Integration ...$integrations)`, `available(): list<Integration>`, `abilities(): list<Ability>`, `directAbilityNames(): list<string>`.
+  - `IntegrationRegistry::__construct(Integration ...$integrations)`, `available(): list<Integration>`, `abilities(): list<Ability>` (nur Abilities verfügbarer Integrationen, die selbst `isAvailable()` true liefern), `directAbilityNames(): list<string>`.
   - `AbilityRegistrar::CATEGORY = 'lit-mcp'`, `__construct(IntegrationRegistry $registry)`, `register(): void`, `registerCategory(): void`, `registerAbilities(): void`, `argsFor(Ability $ability): array`.
 
 - [ ] **Step 1: Failing Tests**
@@ -710,11 +710,23 @@ final class IntegrationRegistryTest extends TestCase
         self::assertSame(['lit/a'], $registry->directAbilityNames());
     }
 
-    private function ability(string $name, bool $direct): Ability
+    public function testUnavailableAbilitiesAreSkippedEvenInAvailableIntegration(): void
+    {
+        $working = $this->ability('lit/a', true);
+        $broken = $this->ability('lit/needs-missing-plugin', true, false);
+
+        $registry = new IntegrationRegistry($this->integration('core', true, [$working, $broken]));
+
+        self::assertSame([$working], $registry->abilities());
+        self::assertSame(['lit/a'], $registry->directAbilityNames());
+    }
+
+    private function ability(string $name, bool $direct, bool $available = true): Ability
     {
         $ability = Mockery::mock(Ability::class);
         $ability->allows('name')->andReturn($name);
         $ability->allows('isDirect')->andReturn($direct);
+        $ability->allows('isAvailable')->andReturn($available);
 
         return $ability;
     }
@@ -772,6 +784,7 @@ final class AbilityRegistrarTest extends TestCase
         $ability->allows('outputSchema')->andReturn(['type' => 'object']);
         $ability->allows('annotations')->andReturn(['readonly' => true]);
         $ability->allows('isDirect')->andReturn(true);
+        $ability->allows('isAvailable')->andReturn(true);
 
         return $ability;
     }
@@ -893,6 +906,13 @@ interface Ability
     /** Whether the ability is listed as its own MCP tool (otherwise only via meta-tools). */
     public function isDirect(): bool;
 
+    /**
+     * Whether the ability can work on this installation (e.g. a required
+     * plugin function or version exists). Unavailable abilities are not
+     * registered at all, so they can never fail at call time.
+     */
+    public function isAvailable(): bool;
+
     /** @param array<string, mixed> $input */
     public function permission(array $input): bool|WP_Error;
 
@@ -1002,7 +1022,11 @@ final class IntegrationRegistry
     {
         $abilities = [];
         foreach ($this->available() as $integration) {
-            array_push($abilities, ...$integration->abilities());
+            foreach ($integration->abilities() as $ability) {
+                if ($ability->isAvailable()) {
+                    $abilities[] = $ability;
+                }
+            }
         }
 
         return $abilities;
@@ -1476,6 +1500,11 @@ final class GuideAbility implements Ability
         return true;
     }
 
+    public function isAvailable(): bool
+    {
+        return true;
+    }
+
     public function permission(array $input): bool|WP_Error
     {
         return true;
@@ -1731,6 +1760,11 @@ final class SiteStatusAbility implements Ability
         return true;
     }
 
+    public function isAvailable(): bool
+    {
+        return true;
+    }
+
     public function permission(array $input): bool|WP_Error
     {
         return true;
@@ -1843,9 +1877,11 @@ final class ServerRegistrarTest extends TestCase
         $direct = Mockery::mock(Ability::class);
         $direct->allows('name')->andReturn('lit/guide');
         $direct->allows('isDirect')->andReturn(true);
+        $direct->allows('isAvailable')->andReturn(true);
         $indirect = Mockery::mock(Ability::class);
         $indirect->allows('name')->andReturn('lit/rare-tool');
         $indirect->allows('isDirect')->andReturn(false);
+        $indirect->allows('isAvailable')->andReturn(true);
 
         $core = Mockery::mock(Integration::class);
         $core->allows('id')->andReturn('core');