feat: generate brain tools skill from OpenAPI; rename db-yaml to postgres (#21)

Cursor skills stay in lockstep with serve handlers. CI checks every bin/
path named in SKILL.md exists.

internal/httpapi/spec.go

@@ -1,5 +1,7 @@
 package httpapi
 
+import "strings"
+
 // Shared HTTP surface: OpenAPI paths and MCP tools are generated from Ops.
 // ServeHTTP must keep the same path strings.
 
@@ -124,3 +126,19 @@ func MCPTools() []MCPTool {
 	}
 	return out
 }
+
+// SkillMarkdown is the Cursor skill fragment generated from Ops/MCPTools.
+func SkillMarkdown() string {
+	var b strings.Builder
+	b.WriteString("# brain HTTP / MCP tools\n\n")
+	b.WriteString("Generated from `internal/httpapi.Ops`. Do not edit by hand.\n\n")
+	b.WriteString("Serve: `bin/brain/serve.go` (`GET /openapi.json`, `POST /mcp`).\n\n")
+	for _, t := range MCPTools() {
+		b.WriteString("- `")
+		b.WriteString(t.Name)
+		b.WriteString("` — ")
+		b.WriteString(t.Description)
+		b.WriteString("\n")
+	}
+	return b.String()
+}