You're reading too much into the context around the words. While we may often speak of preparing for the worst, the words aren't inherently about negative consequences and situations.
The word prepare has a little flair of unwillingness ... On the other hand, the word ready itself denotes some negation and we are not optimistic.
Neither word carries any sense of negativity or unwillingness. We need not prepare only for bad things. For example, I have spent the last week preparing for my vacation and now I'm ready to leave. My vacation isn't "bad" and I'm definitely not unwilling to take it.
In short, if someone is ready, he simply accepts the thing but if someone is prepared he has the alternative way to come out of the situation.
No, this isn't the difference between the two words, because you've missed the appropriate meanings. As above, prepare and ready aren't about dealing with unwanted or unpleasant situations, so your remarks here don't apply.
While the gross meanings of prepared and ready are the same, there is a fine difference between the two. Specifically, one of time and expectations: ready is much more immediate than prepared, and we use it when talking about something expected to occur very soon. If we are prepared, we have gotten ready in advance, or even though something is unlikely to happen.
For example, in the linked article, it talks of being prepared for an Ebola outbreak in the US because one is extremely unlikely. Or examine my vacation example from earlier: I say I'm ready shortly before I leave, but speak of preparing during the time leading up to it.
Suppose I am putting my all money into a new business in a recession and someone warns me about the consequences, do these replies of mine mean the same?
"Don't worry, I'm ready for any consequences"
And...
"Don't worry, I'm prepared for any consequences"
These two are largely the same here. Prepared is more formal, but ready suggests a greater confidence (e.g. the common expression ready for anything) when talking about a hypothetical event. This is because of the subtext of immediacy with ready; even though you're just as prepared regardless of which word you use, if you say ready, people will hear even if this happened right now, I'm prepared to deal with it. When making the same comment with prepared, that right now isn't implied nearly as strongly.
The immediacy of ready over prepared is the reason for the differences that Lucian points out.
When you use "Agreed" as an interjection, it is equivalent to "I agree." It is short for the much more formal "It is agreed."
One way you typically see this done is when speakers are setting the ground rules (or baseline assumptions) before beginning a discussion where there may be some disagreement.
This likely stems from a more formal era where you were making an oral contract with somebody, and the first speaker declares the terms of the contract, and the second speaker is accepting the terms of the contract. "It is agreed that we will proceed the way we have discussed." Over time this has just been shortened to "Agreed" which then morphed into a simple way of showing agreement.
Best Answer
"I'm ready" implies there is something known that you are ready for.
So if you reached the absolute end of a task with no apparent next task, it would not sound right to say "I'm ready."
But if you were at a restaurant with coworkers and had just finished eating and paying the bill, you might say "I'm ready" as a sign that you are done with lunch and are ready to go back to work. Going back to work was the known next step so everyone realizes that's what you are ready to do.
But if you just finished an exercise routine and there was no apparent next step, you wouldn't just say "I'm ready." Ready for what? To take a shower? To go home? To do more pushups? With no known next step, it would sound more natural to say "I'm done."