chore: improve Makefile with help, descriptions and run-sse target

[sd2k]

Example output (minus some colours):

❯ make help
Usage: make [target]

Targets:

build-image                    Build the Docker image.
help                           Print this help message.
lint                           Lint the Go code.
run                            Run the MCP server in stdio mode.
run-sse                        Run the MCP server in SSE mode.
test-all                       Run the Go unit and integration tests.
test                           Run the Go unit tests.