feat(ai-chat): per-MCP-server instructions in the agent system prompt (#180)

Admins can now give each EXTERNAL MCP server a free-text instruction ("how/
when to use this server's tools") that the agent receives in its SYSTEM
PROMPT next to the tool descriptions — porting the built-in SERVER_INSTRUCTIONS
idea to admin-configured servers. Trusted, admin-authored text (like a system
prompt); NON-secret, so unlike headersEnc it IS returned in views/forms.

- Migration: nullable `instructions text` on ai_mcp_servers (old rows = null =
  no guidance). Table type + repo insert/update (blank/whitespace -> null via
  blankToNull). DTO `@MaxLength(4000)`. Service threads it through
  McpServerView/toView.
- mcp-clients: `McpServerInstruction { serverName, toolPrefix, instructions }`
  threaded through the toolset/cache/lease. Guidance is built ONLY for a server
  that actually connected AND contributed >=1 callable tool (the allowlist may
  filter all of them out) AND has non-blank text — so a guide never appears for
  tools the agent cannot call. Cached with the toolset, so an edit is picked up
  next turn via the existing CRUD cache invalidation.
- System prompt: `buildMcpToolingBlock` renders an <mcp_tooling> block INSIDE
  the safety sandwich (after context, before the trailing SAFETY_FRAMEWORK) so
  it informs tool choice but cannot override the rules; each section is headed
  by the server's `prefix_*` namespace. Empty/blank -> block omitted. The
  caller (ai-chat.service) now builds the external toolset BEFORE the prompt and
  passes external.instructions; client-handle lifecycle (close-once) unchanged.
- Client: instructions field in types + a Textarea (autosize, maxLength 4000)
  in the MCP-server form with a namespace-prefix hint; i18n (en/ru).

Tests across every layer (prompt block placement + both SAFETY copies; view
blank->null; buildEntry includes guidance only for connected+>=1-tool+non-blank;
DTO MaxLength; repo + integration round-trip; service wiring). Delegated impl
reviewed (APPROVE); applied the import-type follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/workspace/services/ai-mcp-server-service.ts

@@ -14,6 +14,9 @@ export interface IAiMcpServer {
   enabled: boolean;
   toolAllowlist: string[] | null;
   hasHeaders: boolean;
+  // Admin-authored guidance injected into the agent system prompt (#180).
+  // NON-secret, so it IS returned. Null when no guidance is configured.
+  instructions: string | null;
 }
 
 // Create payload. `headers` is write-only: omit => no auth headers.
@@ -25,6 +28,8 @@ export interface IAiMcpServerCreate {
   // never returned.
   headers?: Record<string, string>;
   toolAllowlist?: string[];
+  // Admin-authored prompt guidance (#180). Blank => stored as null.
+  instructions?: string;
   enabled?: boolean;
 }
 
@@ -39,6 +44,8 @@ export interface IAiMcpServerUpdate {
   url?: string;
   headers?: Record<string, string>;
   toolAllowlist?: string[];
+  // Admin-authored prompt guidance (#180). Absent => unchanged; blank => cleared.
+  instructions?: string;
   enabled?: boolean;
 }