feat(extraction): add Visual Basic .NET language support (.vb) (#648, #639, #170) (#1164)

Vendored patched govindbanura/tree-sitter-vbnet grammar (MIT, ~20-fix patch
+ new external scanner for XML literals and multi-line LINQ continuation;
provenance + rebuild instructions in docs/grammars/tree-sitter-vbnet.md),
vbnet extractor with VB-specific call/index disambiguation, Inherits/
Implements heritage, As New instantiation, events, Declare P/Invoke, and
MustOverride abstract members.

Parse health on five real repos: PolicyPlus 100%, CompactGUI 100%,
staxrip 95.2%, SCrawler 87.2%, PCL 87.5% (upstream grammar: 3-18%).
Retrieval A/B (sonnet): 26-43% faster with 0-5 file reads vs 7-20 without.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.claude/skills/agent-eval/corpus.json

@@ -446,5 +446,28 @@
       "files": "~270",
       "question": "How does an incoming player chat message travel from packet handling to being broadcast to the other connected players? Name the programs on the path in order."
     }
+  ],
+  "VB.NET": [
+    {
+      "name": "policyplus",
+      "repo": "https://github.com/Fleex255/PolicyPlus",
+      "size": "Small",
+      "files": "~94",
+      "question": "When the user toggles a policy to Enabled in the policy-setting editor and clicks OK, how does the new state end up written into the loaded policy source (POL file or registry)? Trace the path from the EditSetting dialog to the concrete write."
+    },
+    {
+      "name": "scrawler",
+      "repo": "https://github.com/AAndyProgram/SCrawler",
+      "size": "Medium",
+      "files": "~320",
+      "question": "When a user download is started for a Reddit user, how does the request flow from the user-level download entry point through the shared downloader base into the Reddit site plugin, and where do downloaded media items get appended to the user's content list?"
+    },
+    {
+      "name": "staxrip",
+      "repo": "https://github.com/staxrip/staxrip",
+      "size": "Medium",
+      "files": "~145",
+      "question": "When a job finishes video encoding, how does staxrip decide which muxer runs and how does the muxer command line get built and executed? Trace from job processing to the mkvmerge invocation."
+    }
   ]
-}
+}