feat(coding-agent/discovery): added Go syntax and API discovery rules

- Added eight new Go-specific rules to the discovery package.
- Registered the new Go rules in the default rule source index.
- Covered the new Go AST matching conditions with test cases in `builtin-defaults.test.ts`.

packages/coding-agent/src/discovery/builtin-rules/go-range-int.md

@@ -0,0 +1,45 @@
+---
+description: "Use for i := range n instead of the C-style for i := 0; i < n; i++ loop (Go 1.22)"
+interruptMode: never
+scope: "tool:edit(*.go), tool:write(*.go)"
+astCondition:
+  - "for $I := 0; $I < $N; $I++ { $$$BODY }"
+---
+
+Go 1.22 lets `for` range over an integer. A plain counting loop from `0` to `n` with step `1` reads better as `for i := range n` (or `for range n` when the index is unused).
+
+## Avoid
+
+```go
+for i := 0; i < n; i++ {
+	use(i)
+}
+
+for i := 0; i < len(s); i++ {
+	use(s[i])
+}
+```
+
+## Use
+
+```go
+for i := range n {
+	use(i)
+}
+
+// Ranging the slice directly is usually clearer than indexing.
+for i := range s {
+	use(s[i])
+}
+
+// Index unused → drop it entirely.
+for range n {
+	tick()
+}
+```
+
+## When it does not apply
+
+- Non-zero start, step other than `++`, or a descending loop (`for i := n - 1; i >= 0; i--`) — keep the explicit form.
+- The body reassigns the loop variable or depends on `i` surviving past the loop.
+- Requires Go 1.22+. If the module's `go` directive is older, keep the classic loop.