feat(graph): add Go language extractor

[0xmohit]

Add code-graph extraction support for Go using tree-sitter-go grammar (https://github.com/tree-sitter/tree-sitter-go).

• New src/graph/extraction/languages/go.ts with GoWalker extracting: structs (class), interfaces, type aliases, functions, methods, generics, imports, calls, struct instantiations, field types.
• New fixture src/graph/__tests__/fixtures/sample.go and 11 tests in extractor-go.test.ts covering declarations, containment, references, generics, and docstrings.
• Register Go in languages/index.ts and grammars.ts (.go extension).
• Vendor tree-sitter-go.wasm from tree-sitter-wasms@0.1.13.

Update docs: docs/code-graph-support.md (Go now Supported), docs/extractors.md (grammar source documentation).

Fix src/graph/extraction/node-id.ts: add guard in getNodeText() for nodes missing startIndex/endIndex (tree-sitter-go comment nodes lack byte offsets when accessed via previousSibling).

Fixes #93

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What

Why

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

Checklist

• Tests pass (npm test)
• No breaking changes (or documented below)
• Tested locally with a real project

Code-graph changes

• This PR targets main
• A linked issue agrees on the bounded extractor/resolver scope
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included
• Any new grammar WASM, extension mapping, extractor, or resolver is registered
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above