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.sql.go

@@ -17736,6 +17736,30 @@ func (q *sqlQuerier) GetChatDesktopEnabled(ctx context.Context) (bool, error) {
 	return enable_desktop, err
 }
 
+const getChatIncludeDefaultSystemPrompt = `-- 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
+`
+
+// 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.
+func (q *sqlQuerier) GetChatIncludeDefaultSystemPrompt(ctx context.Context) (bool, error) {
+	row := q.db.QueryRowContext(ctx, getChatIncludeDefaultSystemPrompt)
+	var include_default_system_prompt bool
+	err := row.Scan(&include_default_system_prompt)
+	return include_default_system_prompt, err
+}
+
 const getChatSystemPrompt = `-- name: GetChatSystemPrompt :one
 SELECT
 	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_system_prompt'), '') :: text AS chat_system_prompt
@@ -17748,6 +17772,37 @@ func (q *sqlQuerier) GetChatSystemPrompt(ctx context.Context) (string, error) {
 	return chat_system_prompt, err
 }
 
+const getChatSystemPromptConfig = `-- 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
+`
+
+type GetChatSystemPromptConfigRow struct {
+	ChatSystemPrompt           string `db:"chat_system_prompt" json:"chat_system_prompt"`
+	IncludeDefaultSystemPrompt bool   `db:"include_default_system_prompt" json:"include_default_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.
+func (q *sqlQuerier) GetChatSystemPromptConfig(ctx context.Context) (GetChatSystemPromptConfigRow, error) {
+	row := q.db.QueryRowContext(ctx, getChatSystemPromptConfig)
+	var i GetChatSystemPromptConfigRow
+	err := row.Scan(&i.ChatSystemPrompt, &i.IncludeDefaultSystemPrompt)
+	return i, err
+}
+
 const getChatTemplateAllowlist = `-- name: GetChatTemplateAllowlist :one
 SELECT
 	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_template_allowlist'), '') :: text AS template_allowlist
@@ -17983,6 +18038,28 @@ func (q *sqlQuerier) UpsertChatDesktopEnabled(ctx context.Context, enableDesktop
 	return err
 }
 
+const upsertChatIncludeDefaultSystemPrompt = `-- name: UpsertChatIncludeDefaultSystemPrompt :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'agents_chat_include_default_system_prompt',
+    CASE
+        WHEN $1::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN $1::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'agents_chat_include_default_system_prompt'
+`
+
+func (q *sqlQuerier) UpsertChatIncludeDefaultSystemPrompt(ctx context.Context, includeDefaultSystemPrompt bool) error {
+	_, err := q.db.ExecContext(ctx, upsertChatIncludeDefaultSystemPrompt, includeDefaultSystemPrompt)
+	return err
+}
+
 const upsertChatSystemPrompt = `-- 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'