feat: separate default and additional Coder Agents system prompts (#23616)

Admins can now control whether the built-in Coder Agents default system
prompt is prepended to their custom instructions, rather than having the
custom prompt silently replace the default.

**Changes:**
- New `include_default_system_prompt` boolean toggle (defaults to `true`
for existing deployments) stored as a site config key — no migration
needed.
- GET `/api/experimental/chats/config/system-prompt` returns the toggle
state, the custom prompt, and a preview of the built-in default.
- PUT persists both the toggle and custom prompt atomically in a single
transaction.
- `resolvedChatSystemPrompt()` composes `[default?, custom?]` joined by
`\n\n`, falling back to the built-in default on DB errors.
- Settings UI adds a Switch toggle with conditional helper text and a
"Preview" button that shows the built-in default prompt via the existing
`TextPreviewDialog`.
- Comprehensive test coverage: 15 subtests covering toggle behavior,
prompt composition matrix, auth boundaries, and integration with chat
creation.

coderd/database/queries/siteconfig.sql

@@ -137,6 +137,24 @@ SELECT
 SELECT
 	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_system_prompt'), '') :: text AS chat_system_prompt;
 
+-- GetChatSystemPromptConfig returns both chat system prompt settings in a
+-- single read to avoid torn reads between separate site-config lookups.
+-- The include-default fallback preserves the legacy behavior where a
+-- non-empty custom prompt implied opting out before the explicit toggle
+-- existed.
+-- name: GetChatSystemPromptConfig :one
+SELECT
+    COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_system_prompt'), '') :: text AS chat_system_prompt,
+    COALESCE(
+        (SELECT value = 'true' FROM site_configs WHERE key = 'agents_chat_include_default_system_prompt'),
+        NOT EXISTS (
+            SELECT 1
+            FROM site_configs
+            WHERE key = 'agents_chat_system_prompt'
+                AND value != ''
+        )
+    ) :: boolean AS include_default_system_prompt;
+
 -- name: UpsertChatSystemPrompt :exec
 INSERT INTO site_configs (key, value) VALUES ('agents_chat_system_prompt', $1)
 ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_system_prompt';
@@ -167,6 +185,38 @@ WHERE site_configs.key = 'agents_desktop_enabled';
 SELECT
 	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_template_allowlist'), '') :: text AS template_allowlist;
 
+-- GetChatIncludeDefaultSystemPrompt preserves the legacy default
+-- for deployments created before the explicit include-default toggle.
+-- When the toggle is unset, a non-empty custom prompt implies false;
+-- otherwise the setting defaults to true.
+-- name: GetChatIncludeDefaultSystemPrompt :one
+SELECT
+    COALESCE(
+        (SELECT value = 'true' FROM site_configs WHERE key = 'agents_chat_include_default_system_prompt'),
+        NOT EXISTS (
+            SELECT 1
+            FROM site_configs
+            WHERE key = 'agents_chat_system_prompt'
+                AND value != ''
+        )
+    ) :: boolean AS include_default_system_prompt;
+
+-- name: UpsertChatIncludeDefaultSystemPrompt :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'agents_chat_include_default_system_prompt',
+    CASE
+        WHEN sqlc.arg(include_default_system_prompt)::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN sqlc.arg(include_default_system_prompt)::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'agents_chat_include_default_system_prompt';
+
 -- name: GetChatWorkspaceTTL :one
 -- Returns the global TTL for chat workspaces as a Go duration string.
 -- Returns "0s" (disabled) when no value has been configured.