feat(tools/gocode): Implement Go code analysis tools

[TobiasGleiter]

Summary

Implement a set of Go code analysis tools for the agent to understand
a codebase. Each tool must be a scalpel — small, focused, and answering
exactly one question. No tool should do more than its name implies.

Rules

• One concern per tool, no exceptions
• Return JSON only
• No tool should wrap or call another tool
• Prefer go/ast and go/packages over shelling out
• If a tool needs more than ~50 lines, it is doing too much

Tools

Orientation

• ListPackages — all packages with import paths
• ListFiles — files in a package with line counts
• GetFileContent — file content with line numbers

Declarations

• ListDeclarations — top-level funcs, types, vars in a file or package
• GetFunctionSignature — signature and doc comment for a named function
• GetStructFields — fields, types, and struct tags for a named type
• GetInterfaceMethods — method set of a named interface

Semantics

• FindImplementors — all types implementing a given interface
• FindUsages — all references to a named symbol
• GetCallGraph — direct callees of a named function, one level deep
• ResolveType — resolved type of a named expression

Documentation

• GetDocComment — doc comment for any named symbol
• RunGoDoc — output of go doc pkg.Symbol

Diagnostics

• RunGoVet — vet output scoped to a package
• RunStaticcheck — staticcheck findings for a package

Navigation

• FindDefinition — definition site of a named symbol
• FindByPattern — regex search scoped to a package or file

Start Here

Implement ListDeclarations, GetFunctionSignature, GetStructFields,
and FindUsages first. These four cover the majority of what an agent
needs to orient itself in an unfamiliar codebase.