refactor: Port C# SDK generation from nextlove to Metalsmith

Replace @seamapi/nextlove-sdk-generator with a Metalsmith + Handlebars
pipeline (via @seamapi/smith), mirroring the javascript-http and python
codegen architecture. The nextlove C# generator (its AST codegen
framework, dataclass serializer, and OpenAPI helpers) is ported verbatim
under codegen/lib as TEMPORARY output-parity workarounds, so the
generated output stays byte-identical after csharpier. A thin, durable
Metalsmith plugin (codegen/lib/csharp.ts) drives the port and emits each
generated file through a passthrough layout.

- codegen/smith.ts: Metalsmith entry point
- codegen/lib/csharp.ts: plugin wrapping the ported generator
- codegen/lib/generate-csharp-sdk.ts, codegen.ts, dataclass.ts,
  schema-modifications.ts, endpoint-rules.ts, types.ts, openapi/*:
  verbatim TEMPORARY ports (grep TEMPORARY to skip in review)
- Keep @seamapi/types pinned at 1.691.0 and change-case at ^4; the
  generated output is tied to both
- Adopt @seamapi/smith eslint/tsconfig config; add eslint.config.ts,
  tsconfig.json, .prettierignore; wire lint/typecheck/format scripts
- Bump Node to 24 (.nvmrc, setup action) and CI test matrix to 22
  (@seamapi/smith requires Node >=20.9)

The generator does not clean the output directory: the previous
generator overwrote without deleting, so the committed output still
contains stale files for schemas removed from the pinned types. Cleaning
would change the output and is deferred (see TODO in smith.ts).

@seamapi/blueprint is installed to satisfy the smith peer dependency but
is not wired into the pipeline: the port iterates the raw OpenAPI spec
for parity, and blueprint 0.55.0 does not parse the pinned types.

Verified: generated output is byte-identical after csharpier 0.29.2;
npm run lint, typecheck, and format all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018oCdmrhyAEX8e9sAFRXZAF

Author: claude

.github/actions/setup/action.yml

@@ -9,7 +9,7 @@ inputs:
   node_version:
     description: The Node.js version.
     required: false
-    default: '18'
+    default: '24'
   registry_url:
     description: The Node.js package registry URL.
     required: false

.github/workflows/check.yml

@@ -17,8 +17,7 @@ jobs:
         os:
           - ubuntu-latest
         node:
-          - '16'
-          - '18'
+          - '22'
         include:
           - os: ubuntu-latest
             os_name: Linux
@@ -40,8 +39,7 @@ jobs:
       fail-fast: false
       matrix:
         node:
-          - '16'
-          - '18'
+          - '22'
     steps:
       - name: Checkout
         uses: actions/checkout@v3
@@ -65,8 +63,7 @@ jobs:
       fail-fast: false
       matrix:
         node:
-          - '16'
-          - '18'
+          - '22'
     steps:
       - name: Checkout
         uses: actions/checkout@v3

.gitignore

@@ -5,6 +5,9 @@
 # For more details, visit the project page:
 # https://github.com/github/gitignore
 
+# Codegen
+CODEGEN.md
+
 # TypeScript build output
 *.d.ts
 *.js

.nvmrc

@@ -1 +1 @@
-lts/hydrogen
+24

.prettierignore

@@ -0,0 +1,4 @@
+# Prettier exceptions
+
+*.hbs
+CODEGEN.md

codegen/content/CODEGEN.md

@@ -0,0 +1 @@
+# Codegen

codegen/index.ts

@@ -0,0 +1 @@
+export default null

codegen/layouts/default.hbs

@@ -0,0 +1 @@
+{{contents}}