Remove the Go reference server; keep only Rust

The Go implementation is preserved in the v1.0.0 tag. Its observable
behaviour stays the API reference: responses recorded from the Go server
live in tools/compatibility_expected.txt, and tools/compatibility.py now
compares the Rust server against them (--update re-records after an
intentional change). The Go SQLite schema is frozen in
tests/fixtures/go_schema_v1.sql for the existing-database test.

Move the web client from internal/httpapi/web to web/, drop the Go CI job
and Go version pin, and update README, AGENTS.md and docs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -9,10 +9,6 @@ env:
   CARGO_TERM_COLOR: always
   # Pin the toolchain to the declared MSRV so CI matches production.
   RUST_TOOLCHAIN: 1.90.0
-  # The differential check compares Rust against net/http behaviour, which
-  # changes between Go releases (e.g. ServeMux redirects 301 -> 307), so the
-  # reference must be built with the same Go the Rust port mirrors.
-  GO_VERSION: "1.27.3"
 
 jobs:
   rust:
@@ -58,39 +54,10 @@ jobs:
           path: target/release/balance-server
           if-no-files-found: error
 
-  go:
-    name: Go (test / vet / build)
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-
-      - name: Install Go
-        uses: actions/setup-go@v5
-        with:
-          go-version: ${{ env.GO_VERSION }}
-
-      - name: Cache Go modules
-        uses: actions/cache@v4
-        with:
-          path: ~/go/pkg/mod
-          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
-          restore-keys: |
-            ${{ runner.os }}-go-
-
-      - name: Test
-        run: go test ./...
-
-      - name: Vet
-        run: go vet ./...
-
-      - name: Build
-        run: go build ./...
-
   compatibility:
-    name: Go ↔ Rust differential + SQLite takeover
+    name: HTTP/JSON reference + SQLite persistence
     runs-on: ubuntu-latest
-    needs: [rust, go]
+    needs: [rust]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -100,11 +67,6 @@ jobs:
         with:
           toolchain: ${{ env.RUST_TOOLCHAIN }}
 
-      - name: Install Go
-        uses: actions/setup-go@v5
-        with:
-          go-version: ${{ env.GO_VERSION }}
-
       - name: Install Python
         uses: actions/setup-python@v5
         with:
@@ -121,10 +83,8 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-cargo-
 
-      - name: Build Rust (debug) and Go reference
-        run: |
-          cargo build --locked
-          go build -o target/integration/go-server ./cmd/balance-server
+      - name: Build Rust (debug)
+        run: cargo build --locked
 
-      - name: Run differential compatibility scenarios
-        run: python3 tools/compatibility.py --rust-binary target/debug/balance-server
+      - name: Compare against the recorded Go reference
+        run: python3 tools/compatibility.py --binary target/debug/balance-server