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/cmd/cmd.go

@@ -51,6 +51,12 @@ func applyGenerationFlags(ctx *lmcli.Context, cmd *cobra.Command) {
 		return ctx.GetModels(), cobra.ShellCompDirectiveDefault
 	})
 
+	// -a, --agent
+	f.StringVarP(&ctx.Config.Defaults.Agent, "agent", "a", ctx.Config.Defaults.Agent, "Which agent to interact with")
+	cmd.RegisterFlagCompletionFunc("agent", func(*cobra.Command, []string, string) ([]string, cobra.ShellCompDirective) {
+		return ctx.GetAgents(), cobra.ShellCompDirectiveDefault
+	})
+
 	// --max-length
 	f.IntVar(ctx.Config.Defaults.MaxTokens, "max-length", *ctx.Config.Defaults.MaxTokens, "Maximum response tokens")
 	// --temperature
@@ -65,14 +71,21 @@ func applyGenerationFlags(ctx *lmcli.Context, cmd *cobra.Command) {
 
 func validateGenerationFlags(ctx *lmcli.Context, cmd *cobra.Command) error {
 	f := cmd.Flags()
-
 	model, err := f.GetString("model")
 	if err != nil {
 		return fmt.Errorf("Error parsing --model: %w", err)
 	}
-	if !slices.Contains(ctx.GetModels(), model) {
+	if model != "" && !slices.Contains(ctx.GetModels(), model) {
 		return fmt.Errorf("Unknown model: %s", model)
 	}
+
+	agent, err := f.GetString("agent")
+	if err != nil {
+		return fmt.Errorf("Error parsing --agent: %w", err)
+	}
+	if agent != "" && !slices.Contains(ctx.GetAgents(), agent) {
+		return fmt.Errorf("Unknown agent: %s", agent)
+	}
 	return nil
 }