Introduce "agents"

An agent is currently a name given to a system prompt and a set of
tools which the agent has access to.

This resolves the previous issue of the set of configured tools being
available in *all* contexts, which wasn't always desired. Tools are now
only available when an agent is explicitly requested using the
`-a/--agent` flag.

Agents are expected to be expanded on: the concept of task-specilized
agents (e.g. coding), the ability to define a set of files an agent
should always have access to for RAG purposes, etc.

Other changes:

- Removes the "tools" top-level config structure (though this is expected
to come back along with the abillity to define custom tools).

- Renamed `pkg/agent` to `pkg/agents`

pkg/tui/views/chat/chat.go

@@ -144,6 +144,12 @@ func Chat(shared shared.Shared) Model {
 	m.replyCursor.Focus()
 
 	system := shared.Ctx.DefaultSystemPrompt()
+
+	agent := shared.Ctx.GetAgent(shared.Ctx.Config.Defaults.Agent)
+	if agent != nil && agent.SystemPrompt != "" {
+		system = agent.SystemPrompt
+	}
+
 	if system != "" {
 		m.messages = api.ApplySystemPrompt(m.messages, system, false)
 	}