agent: Use current shell (#28470)

Release Notes:

- agent: Replace `bash` tool with `terminal` tool which uses the current
shell

---------

Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>

assets/prompts/assistant_system_prompt.hbs

@@ -163,3 +163,8 @@ There are rules that apply to these root directories:
 {{/if}}
 {{/each}}
 {{/if}}
+
+<user_environment>
+Operating System: {{os}} ({{arch}})
+Shell: {{shell}}
+</user_environment>

assets/settings/default.json

@@ -656,7 +656,7 @@
         "name": "Write",
         "enable_all_context_servers": true,
         "tools": {
-          "bash": true,
+          "terminal": true,
           "batch_tool": true,
           "code_actions": true,
           "code_symbols": true,