fix: serve agent instructions via MCP manifest

[nlang]

Problem

Agent instructions were served as an MCP Resource fetched from Vercel on demand.
Clients that don't auto-read Resources (e.g. Claude Code) never saw them.

The instructions field in the server manifest is auto-loaded by clients on connect —
no explicit Resource read needed.

Changes

agent-instructions.md (new file)
Agent-focused workflow guidance: when to use which tool, execution rules, anti-patterns.
Intentionally excludes contributor/architecture docs and formatting conventions that
belong in INSTRUCTIONS.md, not in the agent's context.

src/index.ts

• Read agent-instructions.md at startup via import.meta.url-based path resolution
• Works across npx, global install, local install, and direct node build/index.js
• try/catch so the server starts cleanly even if the file is missing
• Pass content via instructions in McpServer options

The existing Resource endpoint stays — clients that do read Resources still get them.

[vercel]

@nlang is attempting to deploy a commit to the ForLoopCodes' projects Team on Vercel.

A member of the Team first needs to authorize it.