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>

src/feature/code_completion.cpp

@@ -296,7 +296,8 @@ public:
                            llvm::StringRef overload_key,
                            llvm::StringRef signature = {},
                            llvm::StringRef return_type = {},
-                           bool is_snippet = false) {
+                           bool is_snippet = false,
+                           bool is_deprecated = false) {
             if(label.empty()) {
                 return;
             }
@@ -327,6 +328,9 @@ public:
                         }
                         item.label_details = std::move(details);
                     }
+                    if(is_deprecated) {
+                        item.tags = std::vector{protocol::CompletionItemTag::Deprecated};
+                    }
                     overloads.push_back({
                         .item = std::move(item),
                         .score = *score,
@@ -355,6 +359,9 @@ public:
                 }
                 item.label_details = std::move(details);
             }
+            if(is_deprecated) {
+                item.tags = std::vector{protocol::CompletionItemTag::Deprecated};
+            }
             collected.push_back(std::move(item));
         };
 
@@ -431,13 +438,15 @@ public:
 
                     bool has_snippet = !snippet.empty();
                     auto insert = has_snippet ? llvm::StringRef(snippet) : llvm::StringRef(label);
+                    bool deprecated = candidate.Availability == CXAvailability_Deprecated;
                     try_add(label,
                             kind,
                             insert,
                             qualified_name.str(),
                             signature,
                             return_type,
-                            has_snippet);
+                            has_snippet,
+                            deprecated);
                     break;
                 }
             }

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;