feat(extraction): add Erlang language support (.erl/.hrl) (#635, #648) (#1165)

Vendored WhatsApp/tree-sitter-erlang 0.19 (the ELP grammar, ABI 14) with an
Erlang-shaped extractor: multi-clause/multi-arity functions merged into one
symbol, -spec signatures, records with fields, -type/-opaque aliases, -define
macros, -include/-include_lib file edges, and -export-driven visibility.

Modules wrap in a namespace so remote mod:fn(...) calls resolve through the
existing qualified-name matcher as mod::fn with zero resolver changes.
-behaviour declarations link to the behaviour module — gated to namespace
targets only (bare-name fallthrough linked -behaviour(supervisor) to an
unrelated macro constant on emqx). OTP indirection with static targets is
followed: spawn/apply/proc_lib/timer/rpc MFA-argument callees, and
gen_server:call/cast(?MODULE | ?SERVER) to the module's own
handle_call/handle_cast. Var-module dispatch and message sends stay
deliberately unlinked. codegraph_explore also normalizes Erlang-native query
spelling (mod:fn/3, init/2) so named symbols resolve as typed.

Benchmarked on cowboy (189 files), ejabberd (414), emqx (2,447): extraction
PASS on all three; with-codegraph arms reached 2/2/0 file Reads vs 10/5+/19
without, fastest on the largest repo.

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

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

@@ -469,5 +469,28 @@
       "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."
     }
+  ],
+  "Erlang": [
+    {
+      "name": "cowboy",
+      "repo": "https://github.com/ninenines/cowboy",
+      "size": "Small",
+      "files": "~190",
+      "question": "How does an incoming HTTP request travel from cowboy's connection process to a user-defined handler's init/2 callback? Trace the path through the stream handler and middleware chain."
+    },
+    {
+      "name": "ejabberd",
+      "repo": "https://github.com/processone/ejabberd",
+      "size": "Medium",
+      "files": "~410",
+      "question": "When a client sends a chat message, how does the stanza get from the receiving C2S process to the recipient's session on the same node? Trace the path through the router and session manager."
+    },
+    {
+      "name": "emqx",
+      "repo": "https://github.com/emqx/emqx",
+      "size": "Large",
+      "files": "~2450",
+      "question": "How does a PUBLISH packet from an MQTT client reach the sessions of matching subscribers? Trace the flow from the connection/channel layer through the broker's routing to session delivery."
+    }
   ]
-}
+}