feat(completion): mark deprecated symbols with strikethrough (#414)

## Summary
- Check `CXAvailability_Deprecated` on `CodeCompletionResult` and set
`CompletionItemTag::Deprecated`
- Editors render deprecated completions with a strikethrough on the
label

## Test plan
- [x] `DeprecatedTag` — `[[deprecated]]` function gets the tag
- [x] `NotDeprecated` — normal function has no Deprecated tag
- [x] All 491 unit tests pass
- [x] `pixi run format` clean

Stacked on #411.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Code completion now marks deprecated declarations with a deprecated
tag so users can see deprecated items in completion lists.

* **Tests**
* Added unit tests ensuring deprecated declarations produce completion
items with the deprecated tag and non-deprecated items do not.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/feature/code_completion_tests.cpp

@@ -233,6 +233,33 @@ void bar() {
     }
 }
 
+TEST_CASE(DeprecatedTag) {
+    code_complete(R"cpp(
+[[deprecated]] int foooo(int x);
+int z = fo$(pos)
+)cpp");
+
+    auto it = find_item("foooo");
+    ASSERT_TRUE(it != items.end());
+    ASSERT_TRUE(it->tags.has_value());
+    auto& tags = *it->tags;
+    ASSERT_TRUE(std::ranges::find(tags, protocol::CompletionItemTag::Deprecated) != tags.end());
+}
+
+TEST_CASE(NotDeprecated) {
+    code_complete(R"cpp(
+int foooo(int x);
+int z = fo$(pos)
+)cpp");
+
+    auto it = find_item("foooo");
+    ASSERT_TRUE(it != items.end());
+    // Non-deprecated should have no Deprecated tag.
+    ASSERT_TRUE(!it->tags.has_value() ||
+                std::ranges::find(*it->tags, protocol::CompletionItemTag::Deprecated) ==
+                    it->tags->end());
+}
+
 TEST_CASE(NoBundleOverloads) {
     feature::CodeCompletionOptions opts;
     opts.bundle_overloads = false;