feat(mcp): add server instructions to guide efficient prompt delegation

Leverage MCP WithInstructions to tell Claude Chat to send concise
functional prompts instead of writing code or documentation that
Claude Code will rewrite anyway. Reduces token waste significantly.

Author: btouchard

internal/mcp/server.go

@@ -20,13 +20,26 @@ type Deps struct {
 	Version      string
 }
 
+// serverInstructions are returned to the client during MCP initialize.
+// They guide Claude Chat to delegate coding to Claude Code efficiently.
+const serverInstructions = `Herald bridges you (Claude Chat) to Claude Code running on the user's machine.
+
+IMPORTANT — prompt efficiency:
+- Claude Code has FULL access to the codebase, files, and git history. You do NOT need to write code, file contents, or documentation in your prompts.
+- Send concise, functional prompts that describe WHAT to do, not HOW. Claude Code will figure out the implementation.
+- BAD: pasting a full file with modifications, writing code blocks for Claude Code to apply, or drafting documentation to be written.
+- GOOD: "Add rate limiting middleware to the API routes (200 req/min per token)" or "Fix the null pointer in task.go when priority is empty".
+- Think of yourself as a product manager giving clear requirements, not a developer writing code.
+- If the user provides code or file content, summarize the intent instead of forwarding it verbatim.`
+
 // NewServer creates and configures the MCP server with all tools registered.
 func NewServer(deps *Deps) *server.MCPServer {
 	s := server.NewMCPServer(
 		"Herald",
 		deps.Version,
 		server.WithToolCapabilities(true),
 		server.WithLogging(),
+		server.WithInstructions(serverInstructions),
 	)
 
 	registerTools(s, deps)

internal/mcp/tools.go

@@ -22,7 +22,7 @@ func registerTools(s *server.MCPServer, deps *Deps) {
 			mcp.WithDescription("Start a Claude Code task on a project. Returns immediately with a task ID. The task runs asynchronously — use check_task to monitor progress. Tasks typically take 1-10 minutes. Use check_task with wait_seconds=30 to long-poll efficiently instead of polling rapidly."),
 			mcp.WithString("prompt",
 				mcp.Required(),
-				mcp.Description("The task instructions for Claude Code"),
+				mcp.Description("The task instructions for Claude Code. Send concise, functional requirements — describe WHAT to do, not HOW. Do NOT write code, file contents, or documentation here: Claude Code has the full codebase and will implement it. Think clear requirements, not code."),
 			),
 			mcp.WithString("context",
 				mcp.Description("Human-readable context explaining why this task was launched, for tracking across chat sessions"),