Redesign and clean up all icons across Zed (#35856)

- [x] Clean up unused and old icons
- [x] Swap SVG for all in-use icons with the redesigned version
- [x] Document guidelines

Release Notes:

- N/A

crates/agent_ui/src/agent_configuration/manage_profiles_modal.rs

@@ -594,7 +594,7 @@ impl ManageProfilesModal {
                                         .inset(true)
                                         .spacing(ListItemSpacing::Sparse)
                                         .start_slot(
-                                            Icon::new(IconName::Hammer)
+                                            Icon::new(IconName::ToolHammer)
                                                 .size(IconSize::Small)
                                                 .color(Color::Muted),
                                         )
@@ -763,7 +763,7 @@ impl Render for ManageProfilesModal {
                         .pb_1()
                         .child(ProfileModalHeader::new(
                             format!("{profile_name} — Configure MCP Tools"),
-                            Some(IconName::Hammer),
+                            Some(IconName::ToolHammer),
                         ))
                         .child(ListSeparator)
                         .child(tool_picker.clone())