Use make targets in the README development instructions

[ericof]

What

Replaces the raw pnpm commands documented in the README with their make equivalents, and adds the targets that were missing from the Makefile to make that possible.

Changes

README

• The HTTP transport is started with make start.
• The local development clone block uses make install / make build.
• The development commands block is fully converted to make, and now points at make help for the complete target list.
• The troubleshooting entry for a failing local build points at make install.

Makefile — three new targets:

Target	Runs
stdio	pnpm run stdio — the STDIO server from dist/
inspector	npx @modelcontextprotocol/inspector node dist/stdio-server.js
test-all	pnpm test — the whole suite; make test stays unit-only

Note on two dropped commands

The README documented pnpm run dev ("development mode with hot reload") and pnpm run inspector. Neither script exists in package.json, so both lines were stale. They are dropped rather than wrapped in a target:

• make inspector uses the raw npx invocation the README already documented further down, so no capability is lost.
• There is no hot-reload equivalent. If we want one, it needs a real script (e.g. tsx watch src/stdio-server.ts) — happy to add it in a follow-up.

Verification

• make help lists every target.
• make test-all → 186 tests passing across 31 files.

The one remaining pnpm mention in the README is prose explaining that the package manager is only needed for local development — not a command, so it stays.